need one for the rear disc brake setup on my car, and how many lbs of pressure do they usually need ? thanks
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Why would you want residual pressure on disc brakes ?......Bruce
Yeah
Residual pressure valves are used on DRUM brakes because the brake shoes have to move a lot farther than disk pads/pistons to engage the rotating mass, so they came up with a device that holds a little pressure in the drum brake line such that the shoes are held under a bit of pressure at rest, so they are closer to the drums and don’t have to move as far when the brake pedal is pressed.
This is not used if you have 4-wheel disk brakes because all of the disk brake pads need to move about the same distance (almost no distance at all, since they are all riding on the disk rotors all the time). Step on the brake pedal and everything (all pads) move at once and the same distance.
Step on the brake pedal with a disk/drum set-up, and because the disk pads are already riding on the rotors they start braking immediately, while the drums have a way to go before they make contact with the drum. To minimize that lag, you use a residual valve (usually 11 pounds) in the brake line to the rear drums.
A residual valve is never used (IIRC) on a disk-only system.
So, I just heard from Hot Rod Charlie and he uses 2 pound residual valves on some all-disk hot rods if the brakes give a spongy pedal. Who knew...
So for all-disk brakes, you can use a 2 pound residual valve on each brake line if you have a spongy pedal. You can get them from Summit Racing, Speedway Motors or even CIP1.
For a Disk-front/Drum rear, use a 10 or 11 pound residual valve just on the drum line. Same sources.
I had heard that they are used to decrease pressure needed, thanks for the supplier. Darrell
Darrell.....Do you have disc's at all four wheels ? Did someone tell you that you needed to decrease pressure ? If so why / Are you having trouble with your brakes ? Be careful with what you're doing because you could fry your brake pads and discs by leaving residual pressure in the lines. I understand the reason to install a 2psi valve to reduce a "spongy" brake pedal but this could be risky. Especially if the operational tolerance of these valves has a plus or minus specification.
What Gordon told you is good solid info but do be mindful as this is usually not a recommended installation. ......Just trying to help..........Bruce
My pedal is a little spongy, bled brakes twice and don't have any leaks that I can see that would be sucking air in. Was just going to try the valves to see if I could get the pedal a little firmer... maybe its because I've gotten used to the brake booster advantage I get with all of my "modern" daily drivers. The car has 4 wheel disc.
Just ensure you get one with metric threads - many will have American SAE threads forcing you to get adapters.
Here's a 4# one at CB:
https://www.cbperformance.com/product-p/6609.htm
EMPI has a 2# one to -
C13-17-2935 - EMPI - 2 PSI METRIC RESIDUAL PRESSURE VALVE
widebody boy posted:My pedal is a little spongy, bled brakes twice and don't have any leaks that I can see that would be sucking air in. Was just going to try the valves to see if I could get the pedal a little firmer... maybe its because I've gotten used to the brake booster advantage I get with all of my "modern" daily drivers. The car has 4 wheel disc.
You have to remember that old VW brakes (even aftermarket discs) are never going to feel like a modern system. That said, they should feel fairly firm and stop properly. I have heard of guys having to lift each corner of the car up when bleeding lines to get all the air out with some discs.
Drclock's tip of the day: Remove the two caliper mounting bolts, rotate it to the 12:00 O'clock position and bleed that caliper...sometimes this helps.
will try bleeding each caliper that way, will also check the plunger rod to see if there is any adj.
Darrell
Pedal rod all you need is 1/16" of clearance between the point of pedal rest position and contact in the master cylinder
I've found on several occasions, that you need to bleed the brakes with VIGOR. As in pump 3 times hard and quick, hold, bleed, repeat. This dislodges the tiny bubbles that stick to the innards and cause that slight bit of sponge you can't get rid of. Vacuum or pressure bleeders WILL NOT dislodge these tiny bubbles (that was a song, right?).
I have 4 wheel discs, 4 piston front, single piston rear, and the correct(I believe 19mm VW Karmann Ghia MC designed for disc front brakes). I also installed stainless/Teflon lines. There is no sponge, the pedal is very hard and firm, but easily modulated, as demonstrated at LRP braking from 120 to about 60 for turn one, lap after lap. No fade.
Good brakes are essential and excellent to have. The effort isn't as high as you'd think, but it is non-boosted, which we've all become accustomed to.
I think a residual valve is a band-aid, but I could understand a 2 pound version on discs. This would help if the rotors aren't 100% true and tend to knock the pistons back a little.
I think the stainless steel braid covered brake lines are a big improvement in pedal feel. Rubber lines seem to "balloon" a bit when you hit the brakes - causing loss of pressure to the calipers/wheel cylinders. Very old rubber lines are especially bad and often the rubber lines are cracked and can internally collaspe.
With not putting many miles per year on a Speedster, owners have to be aware that brake fluid absorbs moisture in the air (water) just sitting. The parts at the end of the brake lines can rust internally and fail. 2-3 years is good time to renew brake fluid. I once tried silicon brake fluid (DOT 5) - don't do it! It compresses differently (so less pressure generated) than DOT 3 and can eat rubber brake seals.
Agreed Greg. I change my brake and clutch fluid every 2 years on ALL vehicles. In fact, the Spyder is due for a change this year.