Residual Pressure Valve?

[slngsht]

I got some advice today that installing these in the brake line will reduce the initial portion of the brake pedal travel.

The valve basically maintains 2 PSI in the brake line.

 

Anyone tried them?

[scannon]

Is that enough pressure to heat the pads?

[slngsht]

not sure, but apparently there are different valves for different applications... like 10 PSI for drum brakes, etc...

 

I think the theory is that the pedal travel is purely a function moving the slave pistons, so if the 2 psi is used to keep the pad JUST on the rotor, it takes away the first little bit of mushy pedal travel.

 

They were $17 apiece from Jegs, so I'm going to try them.

[Birkin42]

I believe their primary intended purpose is for applications like a formula car where the master cylinders are close to or lower then the brake calipers. This keeps the fluid from bleeding back and pulling the pads away. Don't know if they will do much for a more typical application, but I've never tried. Let us know how they work out.

[slomove]

Admittedly without knowing much about the ins and outs of brake design....I can not see how such minimal constant pressure would help. There should not be anything forcing the pads back from the disks other than the residual wobble and roughness of the disk itself. The constant pressure surely can not overcome this and if it would the pads would be rubbing all the time. I also can not believe that a theoretical aerodynamic effect (like for the flying hard drive heads) can do anything to such heavy friction mechanical parts.

 

Anyway....I never felt the need to improve my brakes once they are bled properly. They bite right away. By far the biggest slack in the system is probably my reaction time until I actually hit the brake with my foot

 

Gert

[slngsht]

My problem isn't reaction time or breaking force. I have to pick my dentures off the windshield when I nail the brakes :jester:

 

My problem is that since the upgrade, the pedal travel is too much, making it impossible to heel and toe. I suspect that my master cylinder bore is too small, so I also ordered a 7/8" bore Tilden master (running 3/4" now).

 

What the guy says makes sense though. if the pads separate just a little bit, you have to move more fluid when you push the brakes to move the slaves back to the point where they make contact, hence the initial soft portion of the pedal travel.

 

If that doesn't do much, I'll stick in the bigger master, and that should take care of it.

[Birkin42]

I'm assuming you've changed the front calipers when you changed the uprights. Are the pads that you are using new or have they been bedded in? Some time you have a long pedal until the pads are seated.

 

Do you know the bore size of the new calipers compared to old? The ratio of the effective area of the old versus the new should give you a good idea if a master cylinder bore change is in order, and if so, how much.

 

Jack

[slngsht]

I'm assuming you've changed the front calipers when you changed the uprights. Are the pads that you are using new or have they been bedded in? Some time you have a long pedal until the pads are seated.

 

Do you know the bore size of the new calipers compared to old? The ratio of the effective area of the old versus the new should give you a good idea if a master cylinder bore change is in order, and if so, how much.

 

Jack

 

Jack, pads and rotors are new, and yes, I went from a '74 Corona front setup to a '04 corvette. I don't know what the bores are exactly, but I've gone from single piston toyota setup to dual pistons on the new setup. I'm fairly certain the new setup has a significantly larger effective bore than the old. This is something that I can measure if the residual pressure valves don't do the trick.

 

They have been bedded in, and I am getting excellent stopping distances from them. Once the pedal goes in the initial couple of inches and i'm able to apply light-to-moderate braking force, the brakes work fantastic. They have just the right level of effort, and the pedal travel isn't that much after that point. The problem I have is that there is much more travel to get to the point where I can apply braking force, and as a result, the pedal has gone too far in for my foot to reach the gas pedal for heel and toe.

[Birkin42]

Are the Vette calipers floating with 2 pistons on 1 side or opposed piston?

If you have a lot of take up travel as well as fairly light pedal force for a lot of stopping power, these are both signs that the M/C bore could be on the small side.

Also, if you have significantly changed the front caliper effective bore area and have non proportionally enlarged the rears, the brake bias front to rear could be off.

I suspect though that most of the pedal take up travel is the result of a combination of pads bedding in, alignment of the caliper to the rotor, how much the seals in the caliper draw the pistons back, how easily the caliper floats (if it is a single sided design) and how much the pads want to pull back via the spring plates etc. It can also be loose wheel bearings or a rotor issue like run out, taper or warpage.

I would think the residual pressure valves would help with some of these.

I would see if your front to rear bias is in the ball park with all the changes you've made.

 

Jack

 

[slngsht]

Are the Vette calipers floating with 2 pistons on 1 side or opposed piston?

 

Yes.

 

 

If you have a lot of take up travel as well as fairly light pedal force for a lot of stopping power, these are both signs that the M/C bore could be on the small side.

 

I'd say the pedal force is just right - not too light.

 

 

Also, if you have significantly changed the front caliper effective bore area and have non proportionally enlarged the rears, the brake bias front to rear could be off.

 

Both have changed. I don't know if the balance is completely right, but I have adjusted it, and I don't think either front or rear locks prematurely.

 

I suspect though that most of the pedal take up travel is the result of a combination of pads bedding in, alignment of the caliper to the rotor, how much the seals in the caliper draw the pistons back, how easily the caliper floats (if it is a single sided design) and how much the pads want to pull back via the spring plates etc. It can also be loose wheel bearings or a rotor issue like run out, taper or warpage.

I would think the residual pressure valves would help with some of these.

I would see if your front to rear bias is in the ball park with all the changes you've made.

 

In the end, I adjusted the pedals some more. The amount I have to lift my foot off the gas is a little more than I like, but at least now I can heel and toe. I will leave it the way it is for 7/7/7.

 

 

Time to stop modding.

Jack

 

[Keith]

I have RPVs in my brake system because of my low master cylinders. Never tried running without them, and I have a nice firm pedal due to fairly aggressive MC sizing I'd start with brake pedal adjustment if there's too much initial travel, but watch you don't go too far. It's possible to take out all the free play and then have your brakes apply themselves as the fluid heats up!