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Question concerning rear disk brake

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Boss

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I'm going with the TSM rear disk kit and I read in Shawns TSM write up that he used a rear adjustable proportioning valve to make it the same as the front (so the rear brakes won't lock up). I also read many places that say that it may be necessary for the rear adjustable prop.
Well, I just called "InLine Tube" so I can get some steel lines from them so I can hook up my rear prop., but the guy told me that a rear adjustable prop. is NOT necessary b/c all it does is DECREASE the pressure going to the rear disk and I need to INCREASE it when converting to disk. Disk requires more pressure than drum.
I thought it was the opposite. He said they sell a prop. valve that goes into the stock place for 4wheel disk conversions for $100!!! Is he trying to scam me?? I didn't buy any lines or adapter fittings from him b/c I'm confused now. I also needed extra lines so I can put in a line lok in the front tires. Anyhow, to yall who know, what's your opinion on this matter. Who is right?? Thanks for any help.
Boss

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Maybe its just me, but if the prop valve was adjustable how would that make it decrease the pressure? Wouldn't you just set the pressure to whatever worked? Wide open it would supply the same pressure front and rear right?
Also rear discs require more pressure than drums but also provide more braking....


Rene

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He is right about the pressure. You have to replace your prop valve in the front with a 4 wheel disk valve OR an adjustable. You still need a pressure bias to the front to prevent sliding the rear just because the front has the engine and much of the weight transers to the front when stopping. An adjustable is good because you can play with it based on your truck's particular weight bias.

Bad Dog

85 K30 CUCV, 350 TBI, TH400, NP205, D60/C14, 4.56
Coming soon: 4" lift, 40" tires, massive cutting, shorter wb and rear overhang.
 
If I understand correctly, Boss was talking about putting the new valve in the back which would be behind the factory valve that would already have lowered the pressure to the rear. Another valve back there could only lower it more, and he needs to higer pressure for the disks..

Bad Dog

85 K30 CUCV, 350 TBI, TH400, NP205, D60/C14, 4.56
Coming soon: 4" lift, 40" tires, massive cutting, shorter wb and rear overhang.<P ID="edit"><FONT SIZE=-1>Edited by BadDog on 03/16/01 02:03 PM (server time).</FONT></P>
 
I'm going to do my TSM convertion next month. I'm going to install it without the adj. valve. If I notice that my rear is locking up before the front, then I'll just buy the Summit one for about $40.

Think about this: You're replacing your old drum brakes with more efficient disc brakes that stop you better. The rear will now want to stop sooner, locking your tires, so you just add a valve that will allow you to reduce pressure to the rear.
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<P ID="edit"><FONT SIZE=-1>Edited by Blazer79 on 03/16/01 11:03 AM (server time).</FONT></P>
 
Thanks guys, I'm still confused, but will get there.
But Rene, Yes, you're right, with the adjustable if its wide open (increased at max), then it will be like an open line, but the guy is saying that it needs to be MORE than regular. He suggested replacing the stock prop. valve (non adjustable) to a new (non adjustable) prop. valve that is made for 4wheel disk. He said the old one only allowed 30% pressure to the rear. The new one will allow 100% or something like that.

Badog,
No, he was talking about REPLACING the stock prop. valve with his new one.
So, I guess I would say, should I even use the adjustable valve that I bought?? (its Stainless Steel brand and costed me $50
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Or should I get a new prop. valve like the one he suggested and just replace my old one and be done with it?? Thanks in advance.
Boss

<A target="_blank" HREF=http://www.boss.coloradok5.com>http://www.boss.coloradok5.com</A>
 
I'd say try it with what you've got. Start with the rear completely open and do some brake tests in a gravel road, with someone standing on the side (not too close, though, for safety reasons). Have that person observe which side locks first when you hit the brakes. If the rear locks up before, you'll be OK with your current parts and just adjust your SS adj valve to reduce pressure a bit from the rear. Repeat this until both sides (front-rear) lock at the same time.

If the front locks before the rear, then make sure the valve is fully open to the rear. If it is, then you'll need a valve as this guy was talking about.

Good luck.

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The guy is right about one thing, discs DO require more line pressure than drums for the same braking action. The way drums are set up, the rotation of the drum pulls one of the shoes into it, increasing the braking action without requiring more hydraulic pressure.

As far as the propo. valves go, I'd go with what Blazer79 suggested and figure it out yourself. Cheaper than throwing money at a problem that may not exist...

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Semper Maintenance!
 
Most calipers have more displacement in the cylinder than a rear wheel cylinder therefore, you need more fluid to the rear than you had with drums. You really ought to change the master cyl to another from a car that had 4w-disks. Maybe a camero or something.

Josh
 
Yeah, that's what I meant by "You have to replace your prop valve in the front with a 4 wheel disk valve OR an adjustable." The stock valve in the front will already be cuttin pressure to the rear *way down* for drums. Even with 4 wheel disk, you want lower pressure in the rear due to weight transer. If it were me, I would replce the factory valve on the front xmemeber with an adjustable one.

Bad Dog

85 K30 CUCV, 350 TBI, TH400, NP205, D60/C14, 4.56
Coming soon: 4" lift, 40" tires, massive cutting, shorter wb and rear overhang.
 
Hmm, just realized, my reply to Rene may have been confusing. The first reply was to you. After I wrote that, I realized that Rene had also replied so the second reply was about what he said.

Bad Dog

85 K30 CUCV, 350 TBI, TH400, NP205, D60/C14, 4.56
Coming soon: 4" lift, 40" tires, massive cutting, shorter wb and rear overhang.
 
Disc brakes in the rear require more force not more line pressure. Because of the larger surface area of the piston in the disc caliper compared to the wheel cylinder the force is greater without any more line pressure. The size of the rear piston is about the same size as the front so at the same line pressure the rear will lock up before the front because of weight transfer to the front. All this means that you put the adj. prop. valve in the rear line to keep them from locking up before the front.

'71 Blazer CST with alot of rust
 
I am no brake expert and I generally agree with you Triaged. The pressure applied to the pad is multiplied by the caliper piston surface area. Typical hydraulic principles. All other things being equal (if they are) with 4 wheel disks you must run less line pressure to the back as we both said. However, disks do require more line pressure than drums, which is what I was talking about earlier. Therefore, the original prop valve for rear drum will not work since the pressure to the back will be insufficient for the disks. A general 4 wheel disk valve for cars will also be unlikely to yield desired results since they will not be set up for the weight characteristics of a lifted Blazer. I plan on eventually doing this to my own project and I will use an adjustable valve replacing the factory valve on the front x-member.

Bad Dog

85 K30 CUCV, 350 TBI, TH400, NP205, D60/C14, 4.56
Coming soon: 4" lift, 40" tires, massive cutting, shorter wb and rear overhang.
 
You need a adj. proportioning valve and a 2lb. residual valve. I got mine from DTS for $75 shipped. Run a new line from the master all the way to your rear block. Somewhere in between {i chose inside the hood compartment} You need to plum in those to pieces. Bleed your brakes and go for a OH SH&#! stop and begin the adjusting of the valve accordingly. Mine work awesome! Before I did this conversion mine would lock up in the rear. After, I had good pedal but not to good grab. I changed the valves and now I got awesome brakes w/35's. Good Luck
 
What's the residual valve for? I thought that was only used when the master cylinder is below the caliper level.
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Could you please explain why you need that?

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The stock drum setup has a 10lb residual in the rear valve that keeps 10lbs of pressure in the lines. When you switch to disk you do not want 10lbs in the rear that creates drag on the rear brakes along with constant wear. The reason the stock setup is 10lbs is so the wheel cylinder doesn't close all the way so you have more instant response when you hit your brakes. With disk you don't need that kind of pressure that is why 2lb.valve is what i am running and also what was recommended by the guys who custom made my d60 w/disk brakes,ARB,35spline Mosers,4:56,1350 yoke, 3" wider than stock, w/ebrake inside the rotors. I couldn't be happier with the setup I am running. Good Luck
 
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