Brake Booster push rod adjustment help

[rugerberetta]

I am hoping to leave on a long trip to the western US in my 71 Fj40 early next week so I took it for a long ride today and when I returned home my front rims were way too hot. I had an issue when I originally installed the new booster and master cylinder where my front brakes would lock on a short drive. I adjusted the push rod and have not noticed the problem again until today which was very hot close to 100 out. Now I think I am having the same issue where the longer I drive the more they will rub until they lock again. It just takes a lot longer than before. Do I need to adjust "shorten" the push rod a fraction more then correct my lack of brakes buy adjusting the wheel cylinders on each wheel or is the push rod length so sensitive that a 1/16 turn will change the brakes from locking on a long trip to a soft pedal and the wheel cylinder adjustment has no bearing on this issue.

My wheel cylinders were also all replace during the build.

To ask my question another way: do you need to adjust the wheel cylinders on each wheel every time you adjust the pushrod length no matter how small the length is changed? or are these too separate issues. If you have the pushrod length correct it will allow the brake fluid to flow back out of the wheel cylinders not matter how they are adjusted.

As you can tell I am confused on this issue please help thanks

 

[Fast Eddy]

The pushrod length has nothing to do with fluid flowing back out. That is the job of the residual valves. The residual valves should hold 10psi in the lines even if the master is removed from the booster/firewall.

It sounds like you may have multiple problems. Soft pedal is air in the lines. Need to pump up the brakes is bad residual valves or misadjusted brakes. Sounds like you have air in the system and have misadjusted the brakes to try to correct it.

Did you bench bleed the master?

 

[rugerberetta]

The brakes have worked fine over the last year on short trips of 20 miles or so but I have not driven when the temp out side is so hot. Master was bench bled lines were bled correctly so I do not think I have air in the lines. The only time I have a soft pedal is when I adjust the pushrod in. I though I read on here that the rod has to come back in far enough to let the fluid empty from the wheel cylinders. When you push the brake pedal fluid fills the wheel cylinders quickly under lots of pressure but it does not exit under much pressure correct? Is the adjustment of the pushrod that sensitive that you have to get it 100% exact or you have issues? thanks

 

[Fast Eddy]

The only time I have a soft pedal is when I adjust the pushrod in.

That doesn't make any sense to me.

 

[kylenine]

rugerberetta : Did you ever figure out your problem? I have run into this same problem as well. I replaced all of my brake lines, cylinders, and my master cylinder. Drove it about 5 miles and the brakes just locked up. I have adjusted the push rod multiple times and it still happens regardless. Any suggestions?

 

[sggoat]

Sounds like the brake adjustment is way off. The Master should have nothing to do with this-it just pushes fluid out when you punch the brakes(that is of course if it returns to neutral--I'm assuming here no vacuum leaks or busted springs). I'm certainly not that familiar with the 71(mine is a 77) but brakes are brakes--Jack the truck up and spin the wheels--are the brakes dragging? If so--adjustment prob., or cylinder issue. If not, you may indeed have a hydraulic issue with the piping or Master.
I had a similar issue with my front discs-the caliper cylinders were worn, causing the pads to stay in contact with the rotor after releasing the brakes--not a lot different with shoe cylinders. They can easily suffer the same binding, and not release fully when the brake pedal is released-keeping the shoe in contact with the drum-generating heat. Also check the shoe springs to be sure they are not stretched/broken.

 

[lostmarbles]

There's more than one pushrod in a "mastercylinder/booster combo" so this thread is confusing to me.

AFAIK it is the pushrod BETWEEN the booster and master being too long that is normally the cause of "brakes applying by themselves when you drive.

I investigated the residual valves on the fluid outlets of my master and I believe they CANNOT (by design) leave any permanent residual pressure in my brake lines. (It is simply not physically possible.) Instead I believe they restrict backflow to maintain SHORT-TERM residual pressure while my pistons in my wheel cylinders are returning to their normal positions after a brake-application (presumably with the purpose of keeping pressure on the seal lips while they're moving so as to help stop leakage at this leakage-prone time). So I personally discount the residual valves as being a likely cause of "brakes holding on by themselves".

But shortening this pushrod (as mentioned in my 1st paragraph) shouldn't ever lead to a "soft pedal". So I assume RugerBrunoBerreta really means "leads to too much pedal travel". However I don't believe it would normally do that either unless things are really "up the poo" or it is being shortened too much.

So I suggest following what the FSM says about setting that pushrod length. (You don't need the SST if you're innovative.)

A soft pedal is normally caused by air trapped somewhere in the fluid system.

And too much pedal travel is normally caused by improper wheel-cylinder adjustment or the linings having worn too far. (That is, assuming you've already correctly set the pushrod freeplay (and this time I mean the pushrod between your pedal and booster) and done the same with the pre-application pedal height and have a proper pedal-return spring installed etc etc.

But I admit that sometimes, especially when the surface wear on brake linings means they no longer match the curvature of their drums properly, you can get the "soft-pedal feel" even when there's no trapped air.



PS. I'm assuming the braking system corresponds to my 1979 build which may or may not be the case.

 

[Fast Eddy]

rugerberetta : Did you ever figure out your problem? I have run into this same problem as well. I replaced all of my brake lines, cylinders, and my master cylinder. Drove it about 5 miles and the brakes just locked up. I have adjusted the push rod multiple times and it still happens regardless. Any suggestions?

Are you sure you have the right residual valve for your brakes? I'm guessing disk brakes? I got a master from Toyota that had a 10 lb residual valve in it and had the problem you describe until I removed it.

 

[igotta40]

"So I suggest following what the FSM says about setting that pushrod length. (You don't need the SST if you're innovative."

What L.M. said... the adjustment if I recall is one tenth of one millimeter. I had the same locking up and hot drums when I finished my rebuild.

You can make an adjustment tool. Once I figured out how, it was simple. I don't have a photo but basically cut a piece of wood (1/2" plywood or one by four) into a 2 1/2 - 3" square. Then trim it so it is the shape of the letter "H". Drill a hole through the middle of the H and put a long bolt (3 - 4") in the hole so it fits snug. Now you have an adjustment tool.

Put the wood a straddle the booster and touch the bolt tip to the rod. Turn the tool over and match the clearance to the master cylinder. Just barely touching, like the thickness of a hair....

 

[lostmarbles]

Are you sure you have the right residual valve for your brakes? I'm guessing disk brakes? I got a master from Toyota that had a 10 lb residual valve in it and had the problem you describe until I removed it.

Were your residual valves similar to this Eddy?:


Or perhaps this?


These are the ones I have in my 1979 drums-all-round BJ40 (that I don't believe can ever leave any permanent residual pressure because of a permanently open hole in the centre of the cap):