71 fj40 m/c with mini truk booster. 4 wheel discs. ford .5 ton fronts, ford car rears. Been working fine for a month now, but as of friday night, if I hit the breaks they work fine , but they don't fully release when I remove my foot from the pedal. A few hits and they're grabbing pretty hard with no pressure on the pedal. If I crack the lines from the m/c to bleed the pressure- it's fine until I hit the breaks again. I removed the booster vacuum hose- no difference, except for manual brakes if I do hit the pedal. You 40 guru's have any ideas or experience with this type of problem?:confused:
Edit: I need to fix this soon so my chrome rotors don't overheat and tarnish!:eek:

Did you remove the 10# residual pressure valves from the MC when you went to disks?

Smashed brake line?

Check the adjustment rod that comes out of the booster behind the master cylinder. If it is adjusted too far out the brakes will stick on...

and check the residual valve...

I also vote residual valve. The "new" fluid in the system may have finally loosened it up and now it's working. Remove it and things should be fine.

Does the mini truck booster have a residual valve in it? The LC one does, but I'm unsure of the mini.

Jeff

where is this 10lb. thingy mo bob? I thought the same thing about the new fluid loosening the rust and now it's working properly (for drums) but didn't know where the thing is ie. master or booster, and a call to wardens yesterday, would indicate there is no valve thingy to remove from the master. I thought about the pushrod, as it was difficult when I installed the setup. when connecting the master to the booster, should they (pushrod and master) just make contact at the last second before they mate fully together? Or how do I get the pushrod the right length? Thanks for the help so far and keep it coming!! Now that I think about it more... the master is a 72 if that makes a difference.?

I don't think you want the push rod touching the MC. I think you want a slight amount of free play (1mm?)

The residual pressure valves are in the master cylinder where the brake lines attach. These 10# valves don't let the line pressure to blead off as fast for drum brakes. With disk brakes, the calipers don't have enough force to push back the 10# of pressure since there are no springs to pull the pads apart like on drum brakes.

This shows the rear valve assy removed for rear disk brakes.....
http://www.birfield.com/~morgan/tech/rdb/day1/024_24a.jpg

This shows the pieces of the valve....the white plastic piece is the residual valve.....
http://www.birfield.com/~morgan/tech/rdb/day1/023_23a.jpg

The residual valve is in the master cylinder, NOT the booster.

Pull the flanged line nuts/fittings off the master cylinder for both front and rear braking systems. Pull the large nut for each, where the lines connect in. (IIRC, 15/16" or so) Inside there you should find a white piece of plastic, relatively round, with a tiny spring on the bottom and 4 tiny holes in it. This is the residual valve. Remove both, toss over right shoulder for luck, and replace all connections. Re-bleed system.

You have now successfully removed the residual valve from your system. A 71 drum brake FJ40 master cylinder should have this valve in both fittings....a 76+ drum/disc combo would only have one for the rear drum brakes.

Yeah, what he said.....or what I said and showed. :D

http://www.ih8mud.com/tech/residvalve.html

Archived forever.....lol

Can you give credit to Morgan since I stole the pics from his site......Thanks Morgan! :D

Heck, the link is probably on your tech site from his rear disk brake swap. LOL

You guys rock!!:beer: :beer: thank you very much, I'll try yanking them out first! Where else can you ask a question and within a day have complete directions and pics to solve the problem??:D :) :usa:

Thanks.....to repay us. you can post your results so we can learn more. Tell us if it worked or not.

will do!

Swapped the leaky clutch master today, all is well. Now, in regards to the brakes... The vacuum booster to pedal was mal- adjusted, it had preload. The booster to master IS adjusted correctly, kinda. There's about an 1/8th" gap between the booster pushrod and the master. The adjuster was too rusty to adjust so I left it as found. Haven't got the brakes bled fully but the problem *seems* to be gone. I also removed the residual valves- they were different from the ones pictured but I got them out of there.

I'll get the brakes finish bled tomorrow and drive it to see if the problem's really is gone. The last pic was of my valve removed. The next is, obviously where it came from.

I was wondering how things were going with that. If there is no free play in the push rod, you will eventually lock up the brakes. I think after you bleed the brakes, you should be good to go.

my pedal sticks down for a couple of seconds, so let me get this straight, if i remove that spring and that plastic thing, itll bounce back like normal after i remove it and re-bleed the system?

this has been annoying me, since i went rear discs, see, i thought it was the return spring, and the lights kept sticking on, and its just been irritating me!!

why do toyota have it in there? what side affects are of removing it?

I don't know what is causing your brake pedle to be sticking down, but I don't think the residual pressure valve could be the cause. That sounds more like something is sticking inside the master cylinder. The springs should be pushing back the brake pedle. See if there is something restricting the brake peddle linkage as well.

There are two different uses for residual valves. The 10 lb valve is used to hold a residual pressure to the drum brakes to keep the shoes out close to the drums giving a higher firmer pedal. The 2 lb valve is used in the disc when the master cylinder is lower than the calipers to prevent back siphoning of the fluid from the master.

bugger, but thanks!!!
so eric, would you recommend removing this valve anyways or what? cause i do have rear discs and not rear drums, and should i go to a wrecking yard and get a 2lb spring and stuff to replace the 10lb one?

This is all just theory and trial and error. Sometimes it works and sometimes it doesn't. If you are having problems with your rear brakes, I would suggest you remove the 10# valve. If not, I would leave it alone.

The 2# valve is mainly used when the MC is LOWER than the calipers so the fluid will not flow back into the MC. Since we don't have street rods, our MCs are above the calipers so we usually don't need the 2# valves. However, sometimes the 2# valves will help.

thanks eric, im thinking the MC may be tired (was new 2 years ago) but it sat for over a year doing nothing, the brakes themselves all work fine and stop the 35s without a drama!!

its probly something else

thanks

rob

I ran into this same problem a few years ago with a 67 FJ, disk rears and a 76 FJ master cylinder. This rig had sat for a year or so and after some other upgrades it was back on the road, every time I drove it more than two miles the rears would start to bind up. after two days of troubleshooting ( everything suggested and more) I found that the fluid return orfice in the master cylinder was plugged. If you disassemble the master and pop off the fluid pots there are two holes in the cast of the master cylinder. the larger is the inlet, and the smaller is the return. if this is plugged by scale it will cause the brakes to bind. Or if the pedal adjustment is set too far in, usually 10mm or more, the plunger will cover the return orfice and will also cause the brakes to bind.

I didn't think about that. Good point.