Brakes locking up - WTF changed?

[haystax]

Brakes on my 40 are locking up once the brake pedal is pushed. Stock '76 front discs w/ 4 Runner calipers, Chevy rear disc swap, 80 series MC, early single diaphragm booster.

I completed the brake swap and upgrade over two years ago, been great up until this spring. Now the MC will not release pressure. Pedal was always a little odd, kinda stiff which led me to believe the booster may not of been a good unit (bought used from mud member who assured it was okay)

MC pushrod was trimmed and adjusted, no drag when initially installed, no prop valve installed at this point either.

Pulling up on the brake pedal doesn't seem to help, pressure stays for a couple days and then eventually bleeds off. I can't seem to get the booster test trick about listening to idle and whatnot from the FSM to make sense either. Has good vacuum and no discernible leaks.

Thinking the booster must be bad, can the linkage come apart internally and allow the pushrod to "grow"?? Thanks for the help, will probably tear into it tonite and see what I can find.

 

[Poser]

When the brakes are locked up, unbolt the master cylinder from the booster, leaving the lines attached, and pull the master forward away from the booster and see if the brakes release....


Post up what you find.

 

[natvet]

mine did this and it was one of the soft lines collapsing(when vacuum created by releasing brake)
and preventing release of pressure......or what poser said

 

[Tankota]

mine did this and it was one of the soft lines collapsing(when vacuum created by releasing brake)
and preventing release of pressure......or what poser said

Friend of mine had this happen on left front rubber brake line a couple years ago. Weird and a pain to figure out but we got it pinpointed and a new line fixed it.


Haystax, are all your brakes locked up or just front or just rear or just one?

 

[haystax]

Poser -

Brakes all release when I pull the MC away from booster. Seem to operate fine by manually actuating the MC piston.

Pedal hangs up when depressed and it takes a strong tug to get it to release. Doesn't seem to change with vacuum disconnected either. All components were new aftermarket parts from Napa when installed. Might have been more like 4 yrs ago. Rubber lines to fronts were new, longest Raybestos I could find. Rears are Downey stainless flex. Only part that is not new is the single stage early booster.

Does it make sense to use an 80 series booster with the 80 MC?? Should I just look for a new, single stage booster? I'm really not sure the booster was ever good to begin with as the pedal has always seemed hard, but much better than before the mods.

Thanks for the ideas

 

[Poser]

You may have more than one issue here.

Poser -

Brakes all release when I pull the MC away from booster. Seem to operate fine by manually actuating the MC piston.

This statement would lead me to believe that the link between the booster and the master cylinder is too long, keeping the master cylinder piston partially engaged, preventing the fluid from returning to the reservoir.


With that said however,

Pedal hangs up when depressed and it takes a strong tug to get it to release. Doesn't seem to change with vacuum disconnected either. Only part that is not new is the single stage early booster.

If the booster is somehow binding up/preventing the actuation rod from being able to properly release, then it is very possible that the booster is your issue.

Does it make sense to use an 80 series booster with the 80 MC?? Should I just look for a new, single stage booster? I'm really not sure the booster was ever good to begin with as the pedal has always seemed hard, but much better than before the mods.

A booster from an 80 series may work; I have not ever removed one/installed one to know that they are interchangeable for certain. I have installed 60 series boosters and mini truck boosters on 40 series when doing disc brake conversions. Most require some modification to either the rod between the master cylinder and the booster, or the pedal and the booster linkage. No big deal however.

 

[Blue77FJ40]

Does the pedal still hang up when the mc was removed from booster? If so, then maybe something is bent or in need of lubrication. You may want to disassemble the swivel points and lube the hell out of it. Is the return spring the correct one and still attached?

If this doesn't apply, then what Poser said.

 

[haystax]

Thanks guys for the suggestions -

Need to pull the booster out tonite, I sprayed lots of WD-40 and Brakleen around but not much help. There is a return spring but who knows if it is stock or not.

I trimmed the pushrod and adjusted it to where I thought it should have too much free play if anything when I originally did the swap - had always worked great.

I'll try to get some more info once I pull the booster - thanks!

 

[haystax]

Update -

So I just pulled the booster off and discovered it has came apart somehow internally and wasn't letting the rod release. As I was carrying it to the shop, the rod came out and I see some metal pieces blocking the hole through the diaphragm.

Not sure what caused this, but it looks like I need to source a new booster. Anyone bought a new one lately? Thanks!