Technical Brake System Issues.

That was with the first master cylinder installation. Do not know for sure with this second master cylinder.

 

Appreciate all the info from everyone. Hope to fix the situation soon as this is getting old. Thanks again, Joe .

 

34Joe- Try backing off the adjusters on the drums. I mean way off. Get some movement in your wheel cylinders. Let those suckers get pushed out so the brake springs can push the fluid back to the master cylinder. Same thing on the front calipers. Put some C clamps on them and push the cylinder in then loosen the C clamps to allow movement. Gently push brake pedal down and back up. Air bubbles can be trapped. Also no mention of a proportioning valve- if you’re using one? Or known as a distribution block. Good Luck!

 

Yeah I've had this issue before, and it sounds like you're using the Corvette style master with the 2 retainer clips on the cap. Sounds like you've nailed down the rod going into the master from the booster since you stated you have .020 clearance. Every one of them I've used comes with a small extension that is needed to get the full stroke of the piston in the master. The last time i had this issue, it was the pushrod from the pedal to the master. I hadn't tightened the jamnut and the pushrod had backed off. I didn't figure it out until the 3rd master cylinder.

 

Appreciate the info, will give it a try. Hard to understand the problem as all was well until I replaced the master cylinder and brake booster, with the exact same parts. .I do not have a proportioning valve in the system .

 

The push rod from the pedal is part of the power booster. When the pedal is connected to the rod, there appears to be 1 inch of play before you feel any resistance. No one has been able to tell me if this is adjustable play or not. Thanks for the info.

 

I have not come across a booster that came with an integral pushrod; always had to fab them to fit the set up I was building. I would definitely check that out; 1" free movement seems out of whack.

 

I wish I could get a confirmation on that as it would help in bringing the brake pedal up causing it to engage sooner. Still the problem is that I did not have any problem until I replaced the booster and master cylinder.

 

My son has a 57 Dodge wagon with an aftermarket brake booster of unknown manufacturer. The "kit" came with the car and we used it because it was all brand new. With the set up he has, the rod between the pedal and the booster is part of the booster.
At one time, there was an adjustment bolt that was part of that rod, and connected to the pivot point on the brake pedal but the bolt was jammed and would not turn. To replace that bolt would require replacing the booster. To adjust the bolt (for the short time it worked) you had to pull the brake pedal out of the bracket that held the pedal and turn the bolt/nut assemble. That adjustment assembly had a 1/8" thick walled steel bushing that was attached to to the pedal as a pivot point with a bolt through the bushing and the pedal It was a really poor design.
My son had a total brake failure when the adjusting bolt between the booster and the pedal broke the threads out of the steel bushing. Fortunately, the damage was a lot less then it could have been, the failure happened as he pulled into my driveway at a low speed. We have since modified the assemble and added back in some adjustability. Gene

 

I feel your pain. I have been struggling with this very problem for a long time. A couple of days ago, I fixed mine.

I have tried to read through all the post this morning. If I understand correctly it seems you have a lot of pedal movement before you get the feeling the brakes are doing anything. In my situation, I had to adjust the freeplay of the pedal so that the master cylinder started working as soon as possible. You need some freeplay but it should be minimal. If it is to tight when the brakes warm and the fluid warms and expands, your brakes will start to drag. I have the same system you described and I adjusted the Heim joint on the end of the push rod that connects my pedal to the booster. By adjusting this Heim I found the length that works best. In my case the change in the length of that push rod by less than 1/4 " made a big difference.

 

There is no good reason not to bleed the unused master cylinder ports. What have you got to loose?

 

The booster pedal push rod is normally part of (integral) the booster control valve and is not adjustable for play, only length to the pedal. It's the output rod that has to be adjusted for length, which you have done.

 

I have the same type of booster, the rod going to the brake pedal from the booster is a threaded rod with a jam nut and is part of the booster. I have an inch of play at the pedal before i start to feel nay type of resistance. Do not know if this is correct or not.

 

Appreciate the info, I have been dealing with this for several months, with no happy ending. I keep replacing parts and ending up at the same spot.

 

Yes there is, as I would end up probably having to power bleed the system again.

 

Appreciate the info.

 

Had the same or similar issue with a get up like this. Turned out that the booster couldn't "share" vacuum with anything else. The PCV valve was tee'd to that source, eliminated that and the skies cleared. Had proper brakes. No, I didn't believe it either, but that's what it was, my conclusion was the small booster was more sensitive to the measure of vacuum available, even though standard checks showed 17 on the gauge. If yours is a dedicated source already I guess disregard.

 

My vacuum source is a dedicated source from the back of the intake manifold.

 

Are there differences in the booster push rod from the original booster you removed?

 

I would reduce the free play more. Maybe even go to the point where you start to get brake drag. If that does improve the brakes then back it off some. At least you would know that the freeplay is not the problem. Sometimes it is just as important to find out something does not work or help as it is to find the source of the problem. You can at least eliminate one possibility.

As a side note I have purchased 4 booster/master cylinder sets (2 stage 7" booster and Corvette style master cylinder) for the car I had trouble fixing. The quality of parts today is terrible. Do not eliminate a part that is new as not being part of the problem just because it is new.

 

All this recent talk about pedal to booster free play has me wondering. Is pedal arm "over center"?

 

I have studied about this too and I thought I might be the only one. The pedal ratio could be right, but if the connection to the rod was not in the right location, the amount of movement would be different. This is a hard concept to explain. It seems to me that things traveling in an arc cause different amounts of movement when, as they travel through their path, they are connected to a lever. Much like the relationship of a crankshaft, connecting rod, and piston. The piston speeds up and slows down, and the leverage of the connecting rod changes. This is pretty heavy thinking and I may easily end up confusing myself.

This probably has nothing to do with fixing the brake problem.

 

Both boosters were identical externally, the last two from the same company.

 

I have driven the car around the development with my left foot resting on the brake pedal eliminating some of the free play and it did not seem to do any harm or heat up the brakes. I do agree on the quality of parts which does not make things any easier. Appreciate the info.

 

It is definitely not over center. The lower tab on the brake pedal is at the four o'clock position at rest an at maximum travel ends up at the 8 o'clock position

 

I understand what you are saying, when my brake pedal is all the way up the lower pedal tab is at 4 position and when the pedal is all the way down it is at the 8 position.

 

Haven’t seen it asked yet, but what brake fluid did you use? Wrong stuff would eat your seals in the new master cylinders.

 

So it's not a situation where the master cylinder isn't getting full stroke then?

 

I am using Valvoline Synthetic Dot 3 & 4 brake fluid. This is the same type I have always used.

 

It appears to be getting a full stroke which I confirmed when I pedal bled the system originally .The system has never bled well using the brake pedal. It likes to be power bled , probably because of the booster.