Brake Master Cylinder Not Releasing (1 Viewer)

[LSUDuke94]

Hi everyone here at IH8MUD - newbie here to the site and to FJ40's.

I have a 1971 FJ40 with a 1F, 3 speed that is stock except it has had a disc brake conversion done by the previous owner. Due to the nasty brake fluid in the reservoirs, I did a complete tear down, cleaning and rebuild of the brake master cylinder. I also replaced brake lines and hoses to all 4 wheels and rebuilt the 4 rear wheel cylinders as well. I did not rebuild the front calipers as they looked good and functioned as desired. I have also installed a new brake booster and set the push rod with a brake booster rod adjustment tool. To make sure not to have any pressure on the piston when the brake pedal is released, I gave the rod a little extra room. The check / residual valve was removed for the front calipers and replaced for the rear as suggested when a front disc brake conversion is done.

Problem - As I drive the vehicle and use the brake pedal the calipers and rear wheel cylinders do not want to release. It seems with very light brake pressure things seem to work Ok. When I push the brake pedal further for harder stops all 4 wheels don't release. Then, as I use the brakes more the pedal becomes solid with very little travel and the vehicle comes to a very quick stop and the calipers and wheel cylinders stay engaged and will not release. I have removed the master cylinder from the brake booster when this happens and the piston is all the way against the snap ring, but the calipers and wheel cylinders will not release.

To release the calipers and wheel cylinders I crack the hard line at the master cylinder that go to the front calipers and when the fluid is released both front brake calipers and 4 rear cylinders release and the vehicle rolls free. This is the only what I can get the wheels free.

I have torn down the master cylinder a second time to check that my installation of the seals, springs and pistons were correct as per the manual and from others on the web, but I get the same result. The only thing I can think of, is that the piston in the middle of the master cylinder is not moving back to allow the fluid to return to the reservoir(s) when the brake pedal is released. I have rebuilt multiple master cylinders for a variety of vehicles ranging from the 60' to the 90's before ABS, and I have never come across this problem before.

Has anyone had this problem before and what did you do to resolve it? I am close to buying a new master cylinder, but I would like to keep the original one.

Thanks for any insight and Help resolving this problem.
Duke

P.S. IH8BrakeFluid

 

[Camotubebender]

Did you use a brake booster and master cylinder that is for front disc brakes?

 

[charliemeyer007]

When you did the seals did you use real red rubber grease? Anything other than that or the correct brake fluid can make the seals swell.

 

[locklaw]

Also, if the rubber lines are old and deteriorated the swell up on the inside allowing fluid to pass to the cylinders but not return. The telltale sign of this happening is when the brakes lock up from use and then are released the next day.

 

[charliemeyer007]

Just because you bought new hoses etc. doesn't guarantee they are good. What is your time worth? You can continue to suffer or spend like $100 for a new proper brake master (disk front drum rear- put the old one in bag/box you can always put it back on later.

G

 

[Subterranean Nik]

Have you tried it without the check/residual valve in? Any chance that is in backwards? Post up a pic when you get a moment.

Appears the build up and eventual lock up (pressure not releasing) would be a dirty, incorrect, etc… “check valve”.

Nik

 

[45Dougal]

I had this on my motorcycle where there was crud blocking up the tiny hole at the bottom of the reservoir stopping fluid getting back up - worth checking that they're clear with a cocktail stick, and you should see fluid returning into the pot when you lift off

 

[Subterranean Nik]

Any updates Duke? Hope you got it sorted.

Nik

 

[LSUDuke94]

Any updates Duke? Hope you got it sorted.

Nik

Nik,
No update. It is still doing the same thing. I am trying to find a new master cylinder, but they are back ordered at all the normal places that sell FJ40 parts. I tried Summit Racing and they are back ordered. Now I am looking to see what other years will fit that are in stock to fit a 1971. You and others asked about various things - the residual valve were removed, I cleaned the cylinder inside and out, I honed it, used the correct fluid, there is no dirt in the reservoir, I bench bled it properly and I have rebuilt master cylinders many times on many vehicles in the past 30+ years. I have never had this happen and was wondering if this has happened to anyone else and what they found as the problem. My last thought is that the bore of the cylinder has pits from the rusty fluid that was in it when I took it apart. I think the pits are big enough and the new brake fluid is getting past the seals. This is allowing the fluid pressure to get between the two pistons and not letting the pistons return to their normal position and the fluid cannot return to the reservoirs.

What other years brake master cylinder will fit on a 1971 FJ40?

Thanks,
Duke