Brake problem... master cylinder maybe?

[mountainexplorer]

I have bled the brake system on my '77 Crew Cab to the point where there is no air in the lines, yet still the pedal goes all the way to the floor.

The pedal has no stiffness at all, yet the brakes all work. They work, but not until the pedal is right at the floor. The pedal acts like theres nothing (as if the lines were wide open) until 3/4 of the way to the floor.

The truck originally had a Hydroboost, Dana 60 front, and 1-ton rear brakes. It no has a Dana 44 front, 3/4 ton 13" rear brakes, and a master cylinder and vacuum brake booster that as working out of another 4x4. I figure that the proportioning valve shouldn't be too far off with all the changes together... but could the master cylinder just be too small or bad?

 

[73k5blazer]

Did you just put this setup in and are having this problem, or did the vacuum setup work before and stopped working? If you just put it in, did you swap out the rod that pushes the master cylinder, I believe they are different lenghts.
Did you bleed the master cylinder when you put it in. They need bench bled before installtion, the air trapped in there will never make it to the bleed screws at the wheels.
Other than that, I would say the Master cylinder is the problem.

 

[k20]

I had this problem a week or so ago after I did some brake work. Ended up being the rear brakes were reallyyyyyyyyyy far out of adjustment. Adjusted them up, now the pedal rides higher than the gas, and I have brakes that would throw you through the windshield.

 

[corradofreek]

def check the rear brake adjustment. that could be the whole problem. also, how is the pressure coming out of the rear wheel cylinder when you bleed it? if it seems real weak, it is very likly that it is the master and the seals are worn and not pushing enough pressure

 

[mountainexplorer]

After I initially changed back over to the vacuum brake booster and master cylinder, I tried just switching the booster and retaining the original master cylinder; however it wouldn't work with the vacuum booster because the rod was different, so I had to change out the master cylinder which came with the booster (which I didn't bench bleed). To my knowledge, I remember both units working fine being in a driving vehicle before.

The brakes kind of worked, but I still had the one ton rear brakes with the wider shoes and drums along with the change in the front from a 60 to a 44, so I figured maybe my proportioning valve is mixed up with the change. I was getting straight fluid through the rear bleeder valves, but the rear brakes just weren't really working. Pedal was still very light; felt like it had some pressure and would change a bit if I pumped the pedal.

I changed the rear and and rear brakes to smaller ones, and now it seems like the rear and fronts are working great in proportion to each other as when to kick in (in relation), just the pedal is even worse off, but it does stop better when at the floor.

When I first bled the rear yesterday with the 3/4 ton rear brakes, the first couple times I opened the valves, it seemed there as alot of pressure. After getting straight fluid through several times, it just went to a solid trickle and the pedal never gained any stiffnes even though I did get some braking power.

I thought about adjusting the rear, but since it feels like the front and rear are working the way they should right now in proportion to each other, it would make the rear lock up before the front if I adjusted them too much, wouldn't it?

 

[k20]

mine 'felt' like they were both working, cause the truck stopped really good and evenly. Just the pedal was soft till it got to the floor. I changed the master cyl thinking that was the prob, didnt do a danged thing. Adjusted rears, like I said, pedal is higher than gas, actually have a parking brake now, and it will throw you forward if you stand on em.

 

[mountainexplorer]

Today, I went ahead and changed the master cylinder. Tried 2 different ones, different styles. Didn't change a thing. I tried adjusting the rears, and all I got to happen was the rear brakes to lock up first before the fronts, and the pedal no matter what I did stayed exectly the same.

It's not any of the brakes themselves or an adjustment, it's not the proportioning valve, it's not the master cylinder, it's not the vacuum booster, and it's not the push rod. What's left? I guess all I can do is go back again and re-bleed everything... there has to be an air pocket somewhere.

 

[camiswelding]

allan...

is it possible the proportioning valve got grit in it... and is sticking... IIRc this is a problem I read about before,,,

next step is to put a pressure the on the system... but Im still suspecting all the changes....
POWER BLEED!!!
cam

 

[mountainexplorer]

Whats the best way to bleed out the whole system efficiently? I'm still stuck doing it the old fashioned way.

How much different would you say the proportioning valve is from a factory '77 1-ton 4x4 Crew Cab in relation to any other... say 1/2 or 3/4 ton 4x4 truck proportioning valve?