Brake Woes

[Blueleader]

I'm fighting a wierd problem with my rig. Basically, it's 4-wheel disc with the chevy truck 1/2 ton calipers on all four corners, vacuum booster from a K5 Blazer, and a master cylinder from a later 70's vette (1 1/8" bore, I think). Wilwood adjustable prop valve in the rear brake circuit, just after the master cylinder. Dual Jamar line locks in the dash (I know... they suck. But they're not leaking). All four calipers and the master cylinder are fresh remans.

Front brakes seem to grab fine. Rears barely do anything. Currently, I have it up on jack stands with the wheels off. At idle in 1st gear (automatic trans), I can barely get the rear rotors to stop spinning. Pedal travel is a bit long, too. I've bled the whole system several times with a vacuum bleeder, the kind that connects to an air compressor. I'm pretty damn sure there's no air in the system. The prop valve is cranked all the way up and if I back it off at all, brakes get worse. Flex lines aren't all that old and appear in good shape. Push rod has very little to no pretravel.

Any thoughts? Maybe try a 2 psi residual valve in the rear line? I ran this same system off of a disc/drum K5 master cylinder before and it was much better than this. The rears still didn't grab all that great, though.

 

[TigTorch]

I believe usually the master has different size pistons for the front and rear systems: do you have the ports reversed at the master cylinder?

 

[leadfoot067]

i had a re-man master cylinder bad out of the box, the piston was not returning all the way when the pedal was let off, causing similar symptoms.did you replace any hard lines? ive had them clogged when made..and x2 on the port orientation for the lines out of the master.

 

[Blueleader]

I think they are right. Front port to the front brakes and rear port to the rears. When I made the brake lines, I searched around on a few websites to confirm that it is correct. You know how internet info can be sometimes, though. The ports are two different sizes with the larger (1/4"?) connected to the rear circuit.

 

[Blueleader]

i had a re-man master cylinder bad out of the box, the piston was not returning all the way when the pedal was let off, causing similar symptoms.did you replace any hard lines? ive had them clogged when made..and x2 on the port orientation for the lines out of the master.

All the hard lines are new steel. I'll have to pull some connections apart and check my flares. It's either a Schucks or Autozone reman so a bad master cylinder is definately possible.

 

[zukkev]

I think they are right. Front port to the front brakes and rear port to the rears. When I made the brake lines, I searched around on a few websites to confirm that it is correct. You know how internet info can be sometimes, though. The ports are two different sizes with the larger (1/4"?) connected to the rear circuit.

might want to double-check this, IIRC the front port(furthest from the booster) is for the rears, and rear port (closest to booster) is for the front; Are the resivoirs in the M/C different sizes???I know on GM trucks/cars the smaller of the two resivoirs is for the rear brakes(usually the front port), and the larger resivoir is for the fronts(rear port)....

 

[nevada]

might want to double-check this, IIRC the front port(furthest from the booster) is for the rears, and rear port (closest to booster) is for the front; Are the resivoirs in the M/C different sizes???I know on GM trucks/cars the smaller of the two resivoirs is for the rear brakes(usually the front port), and the larger resivoir is for the fronts(rear port)....

yeh, sounds like theyre reversed. the bigger res should go to the front brakes. some MC's it makes no difference. try swapping them and see how it works.

you might also have too small a bore MC. that will cause alot of pedal travel, and weak brakes.

 

[zukkev]

.....you might also have too small a bore MC. that will cause alot of pedal travel, and weak brakes.

He said he's got an 1 1/8" bore M/C from a late 70's Vette---I don't know what bore these are off the top of my head, but sounds about right...

 

[leadfoot067]

the vette MC is a 4wheel disk master...it may have both res the same size.i personally run a 1 ton chevy master. not sure of the bore size,but its waay better than the 1/2 ton master i started with.

 

[Binder]

It doesn't matter if the lines are reversed as all four brakes are the same size. If you switch the lines the problem should move to the front.......You say the line locks are in the dash as in high up? I bet it's got a air bubble up there.

 

[Bunk1]

I had a problem like this....It was a chunk of rubber blocking pressure at the residual valve.

 

[zukkev]

It doesn't matter if the lines are reversed as all four brakes are the same size. If you switch the lines the problem should move to the front.......
.

If the M/c he's running is a quick-take up style, and the rear part of the master(front port) is hooked to the front brakes, it'll cause them to start braking really rapidly, but with the rears hooked to the port for the fronts,it'll move a high volume of fluid out the M/c,but still be proportioned thru the prop valve, causing less than desireable performance...I guess we'll wait and see what the results are if he posts them...

 

[zukkev]

.......You say the line locks are in the dash as in high up? I bet it's got a air bubble up there.

I suppose it's possible, but he said he used a vacuum bleeder, and a pressure bleeder trying to remedy this....unless he's doing something wrong, there should be no air...

 

[Blueleader]

It doesn't matter if the lines are reversed as all four brakes are the same size. If you switch the lines the problem should move to the front.......You say the line locks are in the dash as in high up? I bet it's got a air bubble up there.

The line locks are actually just a tad lower than the MC. The lines only slope down by about an inch or two, though. I've sucked two quarts of brake fluid through the system so far with the vacuum bleeder. I even tried unbolting the calipers and tipping the bleeder screws straight up just to make sure I didn't miss an air pocket there. If there is still air trapped somewhere, I don't know how I'll get it out.

I think my next step this weekend will be to try another master cylinder. I'd hate to tear the whole system to pieces over a bad reman. If that doesn't work, I'll check all my flares and lines for obstructions. Next, pull the line lock and the prop valve out of the circuit.

So far I don't see how only swapping the ports will solve anything. I suppose if I do so and the problem moves to the front brakes, then it means the MC is bad. I'd rather just buy another MC.

 

[Blueleader]

http://classicbroncos.com/forums/showthread.php?p=1063165

http://www.chevytalk.org/fusionbb/showtopic.php?tid/201584/

Here's a couple links to guys talking about the vette master cylinder ports. If they are right, my connections should be good.


I'll mess with it this weekend and let you guys know the results. Thanks for all the input. :beer:

 

[zukkev]

Interesting, Would be nice to see what you figure out....I'll go check this info out at work today if I get the chance.....:awesomework:

 

[snowplow]

Sorry to dig this up, but in the OP's first post he made it sound like everyone thought that linelocks suck. Just wondering what the deal is because I was thinking of running some electrics in the front to help me steer around a tire. I do know that your not supposed to have them engaged very long. I heard it would burn up the solenoid.

DC

 

[Blueleader]

Most line locks don't suck. It's just the cheapo kind I have. Mine are a spring loaded deal where you apply the brakes and push in a knob to keep the pressure applied. Same principle as an electric lock but not as reliable.

As for my brake problem, I'm still messing with it. It was getting irritating so I moved on to another project for a while.