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My Buddy has a t bucket with the brake master cyl under the floor. He has residual valves in the lines and a proportioning valve. He has disc on the front and drum on the rear. The problem is since the master is below the floor the brake fluid is running back into the master when the pedal is released. We can pump up a good hard pedal but when released and reapplied its gone. He solved the problem by adjusting the brake rod to apply the pressure a little and not let the fluid return but I don't like this. In my opinion it's asking for trouble. How would you solve this problem? Thanks Jesse
 

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It sounds to me like he doesn't have any residual pressure valves or they are not working.

Residual pressure valves work by stopping brake fluid from draining back into the master cylinder when the pedal is released and this keeps your brake pedal firm. The valves keep the caliper pistons and brake shoes from fully retracting so the brakes stay ready to work - without brake drag.

Two PSI is used for disc brakes and 10 PSI for drum brake systems as drums require more pressure to overcome the return-spring tension.

How much did he have to adjust the master cylinder rod to solve the problem? If it was a lot the master may have some problems.

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He does have residual valves and as you described. The master cyl is fine, it builds pressure. My question is, do the res valves create pressure on the caliper and wheel cyl side while acting like a finger covering the up end of an inverted straw filled with fluid. Meaning, not letting the fluid flow back to the master. The instructions say it doesn't matter where they are located, as long as they are in their respective lines. He says he has adjusted the rod about 1/8 inch. I suspect its just enough to not expose the return ports in the master. More ideas?
 

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I have a T-Bucket with the same setup but no residual presure valves, they are not needed, and do not have any issue. I would be willing to bet his brakes need bleeding. On a lot of t-buckets, especially with front disks, due to the mounts for the calipers, the bleeder valve may not be at the highest point so you still have some trapped air. I have seen this several times. I have removed the calipers, installed a block of wood between the pads and bleed them like that. Its a pain but it works.

if the master cylinder is full and the brake system is full therer would be no room for fluid to "bleed back".

Also did you check to see that the rear brakes are adjusted properly. If you make the actuater rod too long you will cover the returns in the master and the brakes will eventually lock up.
 

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#1 Back the pushrod off and adjust it properly...now.

You may have a bad valve.
In testing brand new residual pressure valves i found a 50% total failure rate. these were a name brand valve also. :(

The RPV works just as you describe.


I have found that those residual pressure valves as recieved new, rarely attain their rated pressure, if they work at all. I made a test rig out of a 0-30 psi gauge and a modified bleeder screw. Screw the guage onto the caliper bleeder hole and apply some pressure to the system. Keep the pressure below your maximum gauge rating.
You should see 2# pressure at the caliper when the brake is fully released. Hence the term, "residual pressure"


Your caliper may be to fault also.
Some metric calipers are called "low drag" and they let the piston retract more than others, pushing fluid back into the M/C. The seal is designed with an extra shoulder to make this happen. I have measured piston retraction in these type of calipers to be.025 to .030" new, a stock non "low drag type" will only retract .002".
That is alot of fluid and requires alot of pedal travel to take up... As much as 3/4" in some setups. This would make it appear that the fluid is "draining back" into the master cylinder.

I don't know of any way to tell other than measuring whether or not you have those calipers.

I have found that installing a 10# RPV will cut pedal travel significantly on these cars so equipped.. Typically the 10# wilwood RPV tests out at 5# anyway. ( if it works at all)

If the pads drag then you have too much residual pressure.

The RPV's can't hurt, and they are not a bad idea, alot of OEM stuff has them installed in stock master cylinders for drum brakes. If a guy uses a drum/drum master cylinder under the floor of a car then the valves are already there doing their job, and you don't know it. :)

Hope this helps,
Mikey
 
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