disc brake kit issue, I've done 4 of em and I'm stumped on this one

Like I said I have an issue with a disc/drum set up.
It's based on a GM 70's single piston disc brake kit.
THe issue is when I step hard on the brakes the rear drums will lock up but the front discs are not, and the truck stops much slower then it should.
I've had to replace the booster, but the rest of the system is factory gm (from what I know, P.O. set up the system)
I've got a dual master with the gm factory proportioning valve.
The pedal and bracketry are all gm with no modifications (correct motion ratio)
There is fluid in the resivours
pedal is firm, not spongy and doesnt' go to the floor
I've bled all 4 brakes, the front discs seem to have plenty of pressure and bleed good.
I've re-greased the slider pins and the pads are in good shape, as are the rotors.
Now I've done 4 disc / drum brake setups on other cars and I though I've seen everything and could figure out most. But I've checked most everything I could think of.

The only question is the proportioning valve, can they go bad? If the drums are locking up and the fronts are not working as good as they should be could it be the lines are switched? I would think that would cause the rears to not work as well.
Any way thoughts and ideas to check are appreciated!

 

Seems like you've got a firm grasp on how they work.

They only advice that I could suggest is that the proportioning valve is bad.

I put an adjustable one on my Pontiac with a disc/drum setup. Having the adjustable proportioning valve allowed me to fine tune the balance between my front discs and rear drums.

 

The proportioning valve may have shuttled to the position of back brakes only, as if there was a failure in the other front brake circuit. I had that problem on a '79 Riviera a couple times and removed the warning light switch from the proportioning valve and pushed the valve the other way, then rebled starting with the fronts first.

Your results may vary.

Charlie

 

Do you have the front and rear brakes plumbed to the correct port on the master cylinder? The front ones should be plumbed up to the one closest to the pedal/rod. This gets activated first.

 

Thanks for the replys
Charlie I'll try that with the valve
I'll also check the plumbing but I think it's right

 

get an adjustable valae from speedway motors ............

 

They sure can, and what you described sounds as if this "could" be the case.

If the valve is faulty, adding an adjustable proportioning valve may not solve the problem. This one may require a brake pressure gauge to determine if you are making the required pressure.

 

check the rubber hoses going to the calipers as they could be collapsing internally. i just went through the entire hydraulic system (drum/drum) on my 69' chevy truck and had to replace the rubber lines. done it many times on disc equipped chevy trucks but this was first on drum equiped vehicle.

 

could the rear drums be way out of adjustment? i.e. the shoes riding super close the the drum, and then when pressure is applied they lock up?

 

I don't think so they only lock up when I really stomp on it but it's still slow to stop.
I'll check the rubber lines I didn't think of that

 

What is correct plumbing from the master cylinder to each end? Closest to master cylinder mount to rear of vehicle and farther to the fronts? Assuming master cylinder piston assembly would apply the rear brakes before the front brakes to keep the vehicle braking in a straight line, like big truck air brake stuff with changes in valve opening pressures from rear to front that I'm more familiar with?

 

I second the vote for plumbing backwards. Put the first line closest to the pedal always to the front and you should see a difference. Proportioning valves do go bad and I always try the brakes at nuetral first and then adjust if needed.


Tim

 

With disc/drum I have been told and have installed the rear brake line to the port closest to the mount. I would assume this could be wrong in a "stept" M/C ...................

 

on a disc/drum master cylinder the fluid reservoirs are the "tell". The small resevoir is drum, big reservoir is disc.

 

Let's say he might have bought the "Corvette style" M/C with both reservoirs being equal? Then what? Checking myself too.

 

It's got a big and little master so I'll check that
It may be the lines are swapped

 

get some literature on the master cylinder. mine was a corvette style master and the front port goes to the front.

 

OK you mean the port closest to the mounting flange goes to front? Remember some of these units are facing backwards under the floor so referencing front to back is important.
I to have the Corvette M/C on many cars and have had to take a few apart for a "look see". They have the same reservoir size, same port thread size, same bore all the way threw so I can't figure out why it would matter which port goes where UNLESS it has something to do with the spring rate on the rubber seals inside.............

 

Check.
Brake caliper hoses will go bad. A bad hose can stop pressure or hold pressure. Braking steering pull is the first indication. If stopping is straight, the hoses are usually ok, usually.
Without power brakes, late model ceramic or metallic type brake pads won't have enough pressure applied to work properly. DONE IT. Could be another problem source.
Pinched steel lines? Check the entire system.

 

i think every one might be making this to complex. i had this issue on a few cars threw the years( always after a brake job). i adjusted the rear drums in every case and it solved both problems the slow braking and the rear drums locking up. most of the time the rear shoes are adjust to far in i.e. to far away from the drum.

hope this helps if not sorry to waste your time.

 

Thanks Jessie il look at that also I checked the fronts but not the backs yet

 

Check wheel bearing adjustment. A loose bearing will move the pads out enough to keep them from properly applying to the rotors. Drive down a straight road and apply the brakes and then quickly release and reapply the brakes. If the fronts hold properly, then the problem is either bearing adjustment or a warped rotor taht is allowing the caliper to retract too far.

 

one disc brake kid i did had similar braking problems. ended up being the supplied hoses were assembled wrong, causing a flap of rubber to block the flow under greater pressure. everything seemed ok bleeding or under light braking.

 

what kind of pressure gage do you use to check pressure?, do they just screw into the bleed screw holes? seems like having four gages would be a good idea around the shop.

ok i think i found me answer

http://www.autoanything.com/brakes/61A3154A0A0.aspx

 

Is M/C under the floor? Do you have residual pressure valves, 10lb for drums 2lb for disc? These may already be installed in your M/C, maybe not. Without these your disc pads could be retracting from the rotor face too far. With rears adjusted up tight you could be applying rears while the front pads never really get to the rotor. Just checking.

 

I just installed some new EBC pads acouple weeks ago. Very similar bedding instructions were included with the pads. I doubt this is Aarons' problem.

 

actually, if you look VERY close at the master cylinder casting, the reservoirs are NOT equal size. the front IS larger, but only slightly. Don't let the bumps in the cap fool you. I installed a BUNCH of these masters while working at the street rod shop, and I'll admit that I hooked a few up backwards before I learned to PAY ATTENTION to the casting.

 

HI ,,Charlie is on the right track i think, there is a rubber covered pin you reset on those valves . mine has it . While some one applies pressure you push or pull the pin to set pressure again. hope this helps,,,,52 csb

 

Hi
Hotrod 35 has hit on the answer "presure residual valve" if master cylinder is mounted below the floor you will need 10ib rear and a 2lb front to be installed in the line. I f master cyl is mounted on firewall you should only require a 10lb valve mounted in the rear brake pipe betwean the m/cyl and the diff.
what happens is that the rear pistons in the wheel cyl return way to much when your foot is not on the brake,so when you applie the brakes, pedal travel is to much and passes the parimiters of the front reservoir in the m/cyl ,hense the rear brakes are working long before the fronts
The residual valve keeps the brake fluid in the rear wheel cyl and slight pressure on the rear brake linnings closer to drums

This solved the same problem on my 1965 elcamino


Gary