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Bypassing the Proportioning valve

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by MudFrog, Nov 7, 2002.

  1. MudFrog

    MudFrog 1/2 ton status

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    I just ordered a 1 ton Master Cylinder and Booster from Wagner, and I need to do something with the PV. Should I bypass the PV all together or should I still run the rears through the PV?

    Do you actually reroute all of the lines?? Or can you just add a little segment of SS hose that will allow you to bypass it?
     
  2. Blue85

    Blue85 Troll Premium Member

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    You need a proportioning valve. Without it you are sending straight MC pressure to all brakes and would almost certainly have way too much in the rear. I think the only difference between the disc/drum and disc/disc MC is in the volume of fluid it's designed to deliver to the rear. The prop valve also provides input for the brake light on the dash to tell you when one of the circuits has lost pressure.

    I'm not sure which prop valve would be best in your case, but I would suspect that the one intended for 1-ton 4-wheel disc would be best.

    The combo valve has two inputs, from the front and rear portions of the MC. There are three outputs, front left, front right and rear. So you can't just bypass it anyway, you would have to use junction blocks or something.
     
  3. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Well since we are talking PV and masters I asume that you have a truck that you did a Disc conversion on the rear and are running into some poor brake performance. Correct me if I'm wrong on my guess. Here is my respose if my guess is correct.

    Most people are finding the Disc/drum Proportion valve causes problems with a Disc/Disc conversion. The Disc/Drum PV delays the rear circuit to prevent lock up. The reaso it does this is Drum brakes are self activating. They need much less pressure to do a simulare amount of braking as a disc. This delay can cause a issue with a Disc/Disc set up.
    Some have bypassed with good results and no further modifications needed. Some are finding a little to much in the rear and adding an adjustable PV in the rear circuit.
    www.inlinetube.com has a Disc/Disc PV as well as an Adjustable.
    Personnaly my hope is without the fatory PV to have excessive braking in the rear. Then I can add the adjustable and bias to the weight of the truck. In some circimstances in offroading being able to increase rear bias would be VERY helpfull. In a situation where you might have to back down a hill increasing rear pressure would help prevent the front from locking prematurly and causing a loss of control. By getting the bias closer to 50/50 you would get more effective and controled braking in reverse.

    The master is biased for more brake power in the front circuit. Depending on the weight on the axles you want somewhere between a 60-40 or 70-30 bias to the front for the street. The master already produces less pressure and less volume on the rear cisrcuit and you can tell that simply by looking at the master and seeing the reservior for the rear is smaller.
    I'm also using the Caddie parking brake calipers. The piston is smaller than the fronts that our trucks use. That should help with the volume issue. It will however mean that it cannot produce as much preassure at the pad surface as a larger piston could. How it will actully pan out I'm not sure. Since it will not require the volume my hope is that it will allow the rear circuit to build a higher pressure sooner and be as effective or more effective than if I used a front caliper that needs more volume. Since the the stock master can't provide near the volume of fluid to the rear that it can to the front it may get into a situation that the master runs out of stroke before it builds pressure. Hope that makes sense.
    I did more reseach and the master from a 77 Caddie appears to be biased to produce more pressure for the rear. Just looking at it I think it may well be a bolt on to the existing booster in our trucks. If I run into an issue with too little rear pressure after the conversion then I may try the caddie master in place of the stock unit.
    I'll be doing the conversion this weekend. I plan to bypass the stock valve because most of the stuff that I have read has had the best results in doing so. typically disc brakes require about 900psi and drums require about 300psi. The PV helps maintain that ballance. The master is putting out more than that and you will need that extra to get the rears to function correctly.
    My local parts store has a good selection of fittings that should do the trick for bypassing. You will need a T fitting for the front. THe front circuit from the master is divided in the PV. The rear you will just need the fittings to join the two lines together.
    I will be doing a write up on this but it will be a couple weeks before I get it out. I will give some updates by monday on what I find.
     
  4. weisel

    weisel 1/2 ton status

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    I may be wrong, but if the pv is bypassed, then if anything should happen to one of the brake lines (like a leak), all the pressure would drain out of the system and leave the vehicle brakeless? Dosn't the pv stop this from happening. My buddy's older truck ('50s) broke a brake line while launching his boat and the whole thing rolled into the lake because he had no pv.
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    You know, Blue85 is contributing to the other PV thread, and I'll head there next, but I wonder if part of the problem is different entirely...

    Something I just thought of, I *believe* is master cylinder specific: front disk "quick take up"

    As I recall from the service manual, the front disks are retracted from the rotor slightly when the brake pedal is released. The drums, don't think this is the case.

    If the cars that were disk/drum AND disk/disk used the same prob valve, I'm thinking the master cylinder is different. (if the take up is caused by the MC) If the rears aren't "taking up" as you let off the brake, would they perhaps be too close to the rotor, thus applying faster?

    Cars have a problem with the disk/drum prop valve of too little braking. The trucks have a problem with the same setup valve, except now its too MUCH braking.

    Also as I recall, disks use much more fluid to move the piston than drums, which means that the rear disks on trucks should work less efficiently unless something else is different, right?

    I'm betting most of the problems can be resolved through the prop valve. I'd think delaying the rear brakes and changing the amount of fluid to them would both be accomplished through the prop valve, with mods.
     
  6. MudFrog

    MudFrog 1/2 ton status

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    Ever since I did my 3/4 ton swap it has been one thing after another with my brakes. I have replaced 4 master cylinders in a year because I was told they were bad by the mechanic I use to powerbled my brakes when I can't get a firm petal.

    I did just do a rear disc conversion because I had one of my drums totally lock up on me (the hardware wasn't even a year old), and I really am not good at working on drum brakes so I opted to go with what I knew...

    So after I swapped in the disc brakes I was able to get a decent pedal from bleeding the master cylinder and gravity bleeding the lines. Well 3 days later I was on my way to work and I almost couldn't stop, the master cylinder was bone dry in both resovoirs. I checked for leaks at all the nipples and lins but the only place that seemed to be leaking was at the bottom of the booster. It looks as if the booster was bleeding. I believe the booster to be the factory original btw. So I opted to go ahead and upgrade the master and booster to 1 ton since I was going to replace both of them anyway.

    While I was doing this swap I knew I needed to do something with the PV as well so I was exploring my options. I would like to keep my PV simply because there have been a lot of times I would have gotten myself into trouble had that little brake light not come on. Technically though I could just swap in a 4 wheel disc PV from a 1 ton and not have to worry about it anymore? I really don't think I would need an adjustable, however if they weren't much more I might would entertain the idea.
     
  7. Thumper

    Thumper 1/2 ton status

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    Well, Im no expert, but I have ran the rear discs for nearly 1 yr now. First, its a 12 bolt, with stock front calipers (no e-brake). When I did the swap, I left everything from the rear T at the rear diff forward in place. Immediate problem: The rear discs were dragging badly, groaning, and heating up hot. I asked around and was told that there is a check valve in either the master cyl. or the combo valve to hold the rear drum shoes in almost contact with the drum so they dont have to move much to brake. No one knew where it was, just that there is one. I checked the MC by removing the lines and peeking in, bleeding it, pressuring it in reverse, and couldnt find any evidence of a check valve in the rear circuit. So, I did some research on the combo valve. Couldnt get any firm info on it containing a check valve either so I opted to bypass the rear lines out of it totally. Of course, as soon as there was no further pressure in the rear half of the PV/CV the light came on and the fronts worked fine. No big deal, I lived with the light for a while. The rear discs also seemed to release and stopped dragging. Awesome. Fast forward about 6 months. I am driving along the trail, head down a rainy slick hill, apply the brakes and the rears lock up but no fronts. A little more testing and I determine that my pressure to the fronts have dropped almost to nothing. I bleed them with no results. I check the calipers. Both good. My determination: the stock CV/PV that I left in, has screwed up due to the rear system being locked out for so long... no proof, but sounds logical. No one can back this up around here either. So I install a T and remove the valve completely. Now I have direct pressure to the rears, and also to the fronts. The fronts work again, although on the recently slippery roads, the rears lock way faster and harder than the fronts. Which is not pleasant on icy streets. My determination: I need to install a adjustable PV in the rear system to cut the rear pressure down so they all lock at the same time or the rear after the fronts slightly. Makes sense... thats my next step. As far as losing a line and losing brakes... seems like it wont happen. About 2 months ago (after full removal of the PV/CV valve) I had one of the metal segments of the caliper line on the rear get caught by a tree and bent. It bent so that the rubber portion was contacting the tire. It rubbed a hole in the line and I put the brakes on and blew the line. The pedal went almost to the floor, but I still had front brakes. They sufficed enough to stop the truck safely... mind you not in a panic situation. So, thats about it for my experiences and input. Good Luck.

    Mike
     
  8. dyeager535

    dyeager535 1 ton status Premium Member

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    Only fault I can see with going to an adjustable prop valve is that GM didn't do so when they went to rear disks.

    SOMETHING is not right with our setups to be running rear disks, but whatever it is, once figured out, will be fixed either via the master cylinder or the combo valve, or both.

    I'm not saying the adjustable prop valve is bad, because you need brakes, I'm saying that we should be able to figure out where the problem is coming from.

    I've got a 1987 Caprice master cylinder I can take apart, but really, since its not rear disk, any differences between it and a disk brake MC will not be discovered.

    I've also got the '87 struck service manual and '85/86 CKGP parts manual, which may show exploded drawings if helpful. That Ford pic above looks pretty similar to ours though...
     
  9. Thumper

    Thumper 1/2 ton status

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    Actually, the ones who started running rear discs were the street rod guys. Quite a few articles on disc conversions all referred me to street rods, and all thier setups required a adjustable valve in the rear. Also many of my mud bogging buddies run one as well. They dont see any trouble with them. But you are right, they have probably figured out how to do all the magic inside the MC nowadays. The limiting factor is the price for us mod guys! A 30.00 adjustable valve... or a 150.00 disc/disc MC... Hmmm no contest for me.

    Mike
     
  10. Troopie's Tonka

    Troopie's Tonka 1/2 ton status

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    ooh that would suck!!!
     
  11. imiceman44

    imiceman44 1 ton status

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    If you guys give me one day to look for the magazine, I can find the aarticle, by the way it wasn't a 4 wheeler magazine, but car craft.
    They did say there is a check ball in the master cylinder, and I remember instructions on where and how to find them.
    I will be back soon.
    And Dorian, this week end I will be looking at that PV that you sent me and will let you know what I think.
     
  12. Thumper

    Thumper 1/2 ton status

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    I read an article about it too. They said to remove the rear brake line fitting from the MC, then there will be another fitting inside the hole that is removeable and will let the check ball come out. Errr... I think actually they said it was a rubber check valve type thing. But my MC has no such thing. When I remove the line, there is noything in the hole but air! So I dunno what they are talking about.

    Mike
     
  13. Michael

    Michael 1/2 ton status

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    OK, here is my report from various suppliers:
    1) Stainless Steel Brakes says:
    Optimal brake pressure varies by vehicle and the type of brakes being used. Disc brakes generally require 1,200 - 1,400 PSI for proper operation, while drum brakes require only 600 - 800 PSI. Other factors which affect pressure include weight distribution, tire/wheel size, and suspension type. For example, a car that is very nose heavy and has narrow rear tires with rear disc brakes may only be able to tolerate 800 PSI before brake lockup occurs. A car that is well balanced and has a wide rear tire with rear disc brakes may tolerate as much as 1,200 PSI before lockup. For this reason, Stainless Steel Brakes recommends an Adjustable Proportioning Valve (right), which allows fine adjustment to optimize the brakes on specific vehicles. The idea is to prevent rear wheel lockup during severe braking.
    2) MP Brakes info page: Brake Setup
    3) From How stuff works page, an intro on how it all works with pics:
    Master cylinder/PV

    If yall need more info it's is close as your computer.....yall really need to leave this site occasionaly and look around some....it's amazing what's out there. /forums/images/graemlins/wink.gif
     
  14. imiceman44

    imiceman44 1 ton status

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    You are right and I often do.
    I start with a search engine and just start typing words.
    Most questions that I answer with a link are done when I see the question /forums/images/graemlins/thumb.gif
    By the way I just remembered:
    when I was talking about the check ball in the MC it was about a drum/drum car. Sorry for the confusion.
    I have a couple of 65's with drum/drum and I was doing the research for them, and it stuck in my mind.
    Now guys all you have to do is get a disc/disc PV or use the adjustable.
     
  15. HarryH3

    HarryH3 1 ton status Author

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    I got my disc/disc master cylinder from NAPA for $28. It's from a 1980 Pontiac Trans-Am, with the optional rear disc brakes. /forums/images/graemlins/cool.gif
     
  16. Blue85

    Blue85 Troll Premium Member

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    </font><blockquote><font class="small">In reply to:</font><hr />
    Only fault I can see with going to an adjustable prop valve is that GM didn't do so when they went to rear disks.


    [/ QUOTE ] That's because an OEM would not use an adjustable valve, they would just use a fixed valve with a different rate. The whole idea behind the adjustable valve is that you don't know exactly how much rear bias you need, so you just keep turning the dial until it's right. As you modify a vehicle, it changes everything and nobody wants to stock 600 different valves.

    Whenever GM used rear discs, they either put on a different (non-adjustable) PV, or just left the regular disc/drum one in and settled for reduced braking power in the rear (usually the former).

    Another thing that we haven't been talking about, but that is important is the size of our rear pistons. The bigger the piston you use, the more rear bias you will have and more fluid you will need. If we have two Blazers with rear disc, both with the same MC and PV, the one with caddy calipers will have less rear bias than the one with stock front calipers.

    The reason newer cars don't have proportioning valves is ABS. You can get away with the front/rear bias being a little off because the electronics should prevent lockup. Not my favorite approach, but good for some drivers. So you just set the MC piston and caliper piston sizes to get it close and then let the ABS controller do the rest. Most of these cars have big front calipers and tiny rear ones, which is setting the bias.
     
  17. Blue85

    Blue85 Troll Premium Member

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    </font><blockquote><font class="small">In reply to:</font><hr />
    If I run into an issue with too little rear pressure after the conversion then I may try the caddie master in place of the stock unit.


    [/ QUOTE ]Most of us find that we have too much bias to the rear after the rear disc swap. This is what makes adding the adjustable prop valve so popular. Some are close enough to live with it. I believe that it depends mostly on which rear calipers you choose and the setup of your truck (height, weight distribution). Yes, drum brakes require less pressure, but a lifted truck also needs less rear brake bias than a stock one. Myself, I have stock front calipers in back, and I have too much rear bias. So either something is wrong with my front circuit or 70/30 is still too much out back, since all the calipers are the same.
     
  18. MudFrog

    MudFrog 1/2 ton status

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    So you have 70/30 and your brakes are still too much in the rear? I guess I'll put my 1 ton MC and Booster in and see how the brakes feel. If the rear is overwhelming I'll look into an adjustable PV.
     
  19. dyeager535

    dyeager535 1 ton status Premium Member

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    Even on stock vehicles you had differences that I don't think were ever compensated for with the master cylinder or proportioning valve.

    Of course I could be wrong, since its not a common topic, but did a straight 6 blazer have a different MC than say a heavy half ton V8 truck of the same year? What about vehicles heavily optioned? (again, say a V6 vs V8, AC, power everything)

    It seems to me that the ONLY time GM adjusted the braking system was when the application involved towing. (IE, bigger front rotors, etc) Although, it WOULD be interesting to see if a mid 80's Caprice with 11" brakes has a different MC/prop valve than a 12" equipped one.

    I figure it would be a heck of a lot "cleaner" (and less to go wrong) to jsut use the stock components, only modify them to compensate for the truck. Once the actual culprit(s) are determined, then appropriate mods could be attempted.
     
  20. Blue85

    Blue85 Troll Premium Member

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    I don't know that I have 70/30, I just know that it's stock proportioning. The only things that have been changed are in the back.
     

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