What's new
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.

Some master cylinder info

Zeus33rd

Zeus33rd

Smarter than you
GMOTM Winner
Joined
Mar 14, 2002
Posts
7,324
Reaction score
16
Location
Grass Valley, CA
My truck is 1 ton- 60/14b. 14b has disc brakes using D44 calipers. At first I did the swap and left the stock K5 master cylinder on there. It worked ok for awhile, although the pedal wasn't as firm as I thought it should be. Then after a couple weeks of sitting I noticed that rear resevoir on the master was empty- It had been leaking right from the master. So it was new master cylinder time. Went to get one and figured I'd upgrade to the 3/4 ton master. First off, the 3/4 ton master looks waaaay different that the K5 one. When I asked the parts guy for what I wanted, he asked if it was the JB6 or JB7 system. I didn't know, so I asked him to bring them both out. One had both resevoirs the same size, the other one, one was bigger than the other. The piston on the one with different sized resivoirs was bigger so thats the one I got. Can't remember if that was the JB6 or JB7 though. /forums/images/graemlins/thinking.gif Anyways, after wearing my arm out for half an hour bench bleedign the thing, I bolted it on, an oh man, what a HUGE difference! Pedal is solid as a rock and the truck stops as good, if not better, than my DD '94 F-250. I can leave skid marks with the 42's on command. So those that are running the 1/2 ton/K5 masters with the disc brake 14b, might wanna try upgrading to the 3/4 ton master. Made a big difference for me.
 
You still have the stock vaccum booster I assume?
Hmmmm...my Crew cab has JB6 and I think it has the different size resevoirs, I'll confirm tonight. So if you were askin for a 3/4T and the code is the same is my 1T, I guess that would confirm that the 3/4T and 1T masters are the same? I wonder if there is a price difference between what you ask for, even though its the same part number.

I also found that two different hydroboost systems I had had differnt size pistons also. I cant remember if the smaller one had the same size resevoirs or not. I'll check on that tonight too. But both were from 1ton hydro systems.

Hmmm...wonder how much difference you would see if you had the hydroboost rather than vacuum?
And if none, is it even worth the hassle to change over the whole hydroboost unit, rather than just the m/c? /forums/images/graemlins/thinking.gif
 
Yeah, stock vacumn booster.

[ QUOTE ]
So if you were askin for a 3/4T and the code is the same is my 1T, I guess that would confirm that the 3/4T and 1T masters are the same?

[/ QUOTE ]

That sounds correct to me.

[ QUOTE ]
Hmmm...wonder how much difference you would see if you had the hydroboost rather than vacuum?


[/ QUOTE ]

I don't really think it's worth the trouble to go to hydroboost on a light K5 like truck. My gramps has an '83 K20 with the 6.2 that has hydroboost. It doesn't stop any better than my truck. Plus with hydroboost, if your power steering belt goes, so do your brakes. Just another thing to worry about.
 
Either one makes a difference I have the one with the same size resevoir and it was night and day the difference it made I have the stock brake booster too.
 
[ QUOTE ]
I have a 1-Ton Crew Cab, and my brake system is an JB8.

[/ QUOTE ]

Yours is a '90 isn't it? I asked the parts guy for '76 model stuff. JB6 and JB7 only perhaps? Was there a JB8 in '76?
 
Actually, now that I think about it, I believe my crew cab does have the JB8, and not the JB6.

Any reason you asked for 1976 model stuff as opposed to the 80's models? just curious
 
Another question. Did you happen to look at the piston where the rod sits? Cause on a few different masters that I looked at, that part was different. On one, the rod would only sit about 1/8" in, basically just a divet in the piston, and the other the rod would go into the piston a couple of inches.
Hell, maybe I'll just thow on the master cylinder and screw the other part if it works so well. Alot less hassle, thats for damn sure /forums/images/graemlins/thumb.gif
 
I upgraded to a 1-ton Master Cylinder and Booster after my disc swap. I could not get much pedal and I went through 3 stock Master Cylinders. Finally I ordered the one ton stuff. It did require some modifications but it has worked flawlessly since.. *Knock* *Knock* <-- Me knocking on wood /forums/images/graemlins/ignore.gif
 
[ QUOTE ]
Anyways, after wearing my arm out for half an hour bench bleedign the thing, I bolted it on, an oh man, what a HUGE difference! Pedal is solid as a rock and the truck stops as good, if not better, than my DD '94 F-250.

[/ QUOTE ]

Not to ruffle any feathers, but if your master cylinder was leaking in the first place (which you say it was) you would never get a firm pedal, so you don't have an accurate baseline to compare to. Any time you bleed a used master cylinder, you run an EXTREMElY high risk of it failing. From what I've seen, I'd say about a 75% chance.

I'm sure it made a difference for you, but how much is not going to be clear.

I had a "light" pedal too after a brake swap, (well, swapped axles) and after about four months, the master cylinder finally failed. Big difference in pedal effort/effectiveness with the "exact same" replacement piece.

Anyways, my $.02
 
[ QUOTE ]
Yours is a '90 isn't it? I asked the parts guy for '76 model stuff. JB6 and JB7 only perhaps? Was there a JB8 in '76?

[/ QUOTE ]

Actualy it is a 91, but your guess was close enough.

This is just a guess, but I think JB6 is 1/2 ton vacuum boost brakes, JB7 is 3/4 ton hydroboost, and JB8 is 1 ton hydroboost. I am not sure if there was an JB8 in 76. I know there wher definitly 1 ton trucks in 76 because I owned one.Some people say they do not like hydroboost, but I think it rules. From some of the posts I have seen in the past Hydroboost does not play well with modified high volume/high pressure steering pumps.

When Tim sees this post, he will be able to straighten it out.
 
JB7 is vacuum booster apparently for HD 3/4. That is what I gathered from the info in the post I had about it about a week ago. And I just replaced the master/booster and a caliper on my 3/4 Crewcab. They are the JB7 parts.
 
The difference in 3/4 ton brake systems is based on the size of the front calipers and the rear brakes.

3/4 tons with the 1/2 ton front calipers and 11" rear drums get one brake system, I'm assuming JB6, but I never remember. These will use 52s for front pads and I believe 273s for rear shoes.

3/4 tons with the slightly larger front caliper piston (yes, there is a such thing for both 10 bolt and D44 calipers guys, they bolt up the same but the call for different brake pads and the bigger ones do have a bigger piston) and 13" rear brakes get the bigger system, I'm assuming JB7. This system will use 154 front pads and I'm not sure on the rear shoes.

1 tons obviously have the Dana 60 front so huge front calipers, plus they have 13x3.5" brakes where the 3/4 tons have 13x2.5". This is yet another master cylinder to operate them.

What brake booster you run has absolutely nothing to do with what master cylinder is required. All 1 tons have hydroboost from the factory though.

Ok, now my opinion:

I don't think that hydroboost will improve stopping performance on anything. Power assist really has little or nothing to do with actual braking force. It's probably easier to stomp on the brakes with the hydroboost unit (that's what I've found anyway) and that is probably why you guys think it stops better. It takes a lot of force to move those big pistons in the calipers and to force enough fluid to move those big wheel cylinders in the rear. That is why GM used Hydroboost on all 1 ton trucks if you ask me.

I run a booster off a C10 on mine because I didn't want to buy one when mine crapped out. It is far smaller than the one my truck came with from the factory, but I have found that it has plenty of power assist for my liking, and I can lock my brakes up as well even with my bum knee where I can hardly push the pedal some days.
 
[ QUOTE ]
Any time you bleed a used master cylinder

[/ QUOTE ]

I never bled the used master. I know better than that. /forums/images/graemlins/tongue.gif I was refering to bench bleeding the new one.

And , yes your probably right. The pedal probably would have firmed up with another K5 master cylinder. Mushy pedal due to the leak most likely. But if I can upgrade to a bigger, beefier part, that will push more fluid to waaaaay larger front calipers, and an obviously larger than stock wheel cylinders, rear system with calipers, then why not? As for not having an accurate baseline, well, I know what a brake pedal is supposed to feel like, so thats my basline. hehe

[ QUOTE ]
Any reason you asked for 1976 model stuff as opposed to the 80's models? just curious


[/ QUOTE ]

[ QUOTE ]
Another question. Did you happen to look at the piston where the rod sits? Cause on a few different masters that I looked at, that part was different. On one, the rod would only sit about 1/8" in, basically just a divet in the piston, and the other the rod would go into the piston a couple of inches.
Hell, maybe I'll just thow on the master cylinder and screw the other part if it works so well. Alot less hassle, thats for damn sure


[/ QUOTE ]

Asked for '76 model stuff mostly cause my truck is a '76. Just the first number that poppedinto my head. /forums/images/graemlins/thinking.gif

All the masters I looked at were like the first example you described. Basically just a divit for the rod to sit in.

Here's another thing I noticed. I asked the parts guy for a master for a '76 K20. He then asked me if it was JB6 or 7. I didn't know so I asked for both to brought out. Like I said above, one had to equal sized resevoirs, the other had one large, one small. The one with equal sized resevoirs was also an inch or more taller than the other. Niether one of the ones he brought out looked anything like the stock K5 master that was still on my truck. So I asked him to look up a master for a '76 K5. Different part # again. He brought it out, and it matched the one that was on my truck. So there are two different masters listed for the 3/4 ton, and a different one for the K5. WTF? Which is JB6, JB7 etc? Guess it doens't matter, and the answert is probably right here infront of me, but I still can't see it. /forums/images/graemlins/yikes.gif
 
When I swapped my 1/2ton m/c on my burb for a 3/4ton m/c, I got the one for the JB7. Don't know why I did it that way, maybe just because the JB7 are the bigger brakes(bigger is better theory /forums/images/graemlins/dunno.gif )But my burb is a 75 if that helps /forums/images/graemlins/thumb.gif
 
[ QUOTE ]
I never bled the used master. I know better than that.

[/ QUOTE ]

I gathered from your post that you had done an axle swap as well...so you didn't do anything but gravity or speed bleed the rears then.
 
[ QUOTE ]


I don't really think it's worth the trouble to go to hydroboost on a light K5 like truck. My gramps has an '83 K20 with the 6.2 that has hydroboost. It doesn't stop any better than my truck. Plus with hydroboost, if your power steering belt goes, so do your brakes. Just another thing to worry about.

[/ QUOTE ]
thats why im removing the hydroboost system from the 81 i just got, gonna swap to vacuum,

its a half ton diesel, i dont see any point in having hydro in it, not sure what GM was up to in the 78-85 years ?

good luck
 
Diesel's have hydroboost because they create no vacuum due to the lack of throttle blades. Those trucks do have a vacuum pump, but not enough volume to keep up with a brake booster IMO.

John
 
[ QUOTE ]
[ QUOTE ]
I never bled the used master. I know better than that.

[/ QUOTE ]

I gathered from your post that you had done an axle swap as well...so you didn't do anything but gravity or speed bleed the rears then.

[/ QUOTE ]

Yes, it's 1 ton, so I did do the axle swap...

I built a bleeder thing out of a 1 gallon sprayer and used that...Although I'm unsure of what your gettting at... /forums/images/graemlins/thinking.gif
 
You must log in or register to reply here.

Latest Posts

Top Bottom