Brake Booster issue question

[sandcobra164]

A friend of mine has a truck that develops an issue during a long 60 plus mile drive. The brake pedal will get hard to the point that you cannot press on the brake pedal. The truck has a new master cylinder, new short airpack, fresh DOT 5 in the lines, new wheel cylinder's all the way around, etc. We were traveling back from the GA Rally today and he came to a 4 way stop and he creeped through a stop sign. We re-grouped and after draining the air tanks and bleeding the intermediate axle and front axle, the brakes were fine again. I don't ask too many questions but we've put a whole new brake system on this truck and I'm curious as to what we could have possibly missed. I will add that this truck had the same issues before we put the new airpack on and that was our "final solution" to that issue. It only builds about 100 psi instead of the usual 120 psi but I don't think that has anything to do with it.

 

[gringeltaube]

Start by re-adjusting (make a tad shorter) the MC push-rod.



G.

 

[sandcobra164]

Start by re-adjusting (make a tad shorter) the MC push-rod.

Yes sir!!!

 

[rustystud]

Start by re-adjusting (make a tad shorter) the MC push-rod.



G.

It does sound as if the brake fluid is not fully releasing back into the master cylinder. Good call Gerhard.

 

[sandcobra164]

When we installed the MC, we did have a low pedal after bleeding so we adjusted the rod a little longer. It still has a slightly low pedal but when the pedal gets stiff, it basically has no brakes unless you stand up on top of the pedal and then it has marginal brakes. It made the 180 mile drive up just fine and made it about 110 miles back before it had it's issue and it went on quite a few trail rides at Durhamtown before the issue showed up again. Does that still sound like MC adjustment and if so, we'll certainly try it. I'll add that the brakes never drag on this truck, we're having just the opposite issue. The brakes don't apply when it gets in this mood.

 

[mobileauto2000]

I had a similar thing happen to me. I ended up being the master cylinder. The reservoir had rusted above the fluid level. The rust flaked off and jammed between the piston and the piston cylinder wall so it would not move. A soaking in rust remover and a rebuild fixed it. It was a bear to remove the piston even with compressed air and BP power blaster.

My thought is could a containment have got in the new master cylinder or could some rust have formed where it was not visible in the reservoir?

Easy to test, break the line coming of the master cylinder lose while someone pushes down on the pedal. If it releases it's not the mc.

 

[Floridianson]

As said check free play. As said check the second port that is in the bottom of the Master. As said next time it happens take off the rear service glad hand and see if there is a large release of air. This I believe would be an air pack problem. If the air does not exhaust then the back pressure will hold the shuttle valve closed and not allow it to come back to the charge pisition from the master.

 

[M543A2]

We have had this happen when the rear port in the bottom of the master is restricted or plugged. There are two holes in the bottom, a larger one and a smaller one to the rear of it. The smaller one got plugged and we got the hard pedal you are describing.

 

[M543A2]

I should have mentioned---we used a small wire to clear the port.