An RPV maintains 8-16 psi in the rear drum system to overcome the shoe pull-back springs and allows the shoes to stay out close to the drums for instant response to fluid pressure. Racers changed from discs to drums for weight reduction. Your theory on drag applies to gravity.

Rhetorical: Should air be able to enter a wheel cylinder with good seals?

Some people are receiving MCs with no RPVs. Why? Ask the vendor, they're supposed to know what they are. A vendor should supply you with the proper MC if you state it's a disc/drum system. RPVs come in an MC rebuild kit but you should not have to install them yourself if the vendor has a clue.

More information is in this thread http://www.camaros.org/forum/index.php?topic=2938.0 Other correct info on brake components can be found at CRG also. The vendors refer to parts incorrectly. Pay attention to what John Z writes.

 

An RPV maintains 8-16 psi in the rear drum system to overcome the shoe pull-back springs and allows the shoes to stay out close to the drums for instant response to fluid pressure.

NO. NOT TRUE.

The residual pressure does NOT overcome the shoe return springs. The shoes ALWAYS RETURN TO THE ANCHOR PIN.

Adjusting the shoes for proper shoe-to-drum clearance is done by the BRAKE ADJUSTER not by the residual pressure valve.

The whole point of a metering valve in the front disc brake circuit is to hold-off pressure to the discs until rear brake pressure is high enough to overcome return spring tension. 16 psi is not enough to do this.

Please stop posting this misinformation.

Rhetorical: Should air be able to enter a wheel cylinder with good seals?

Define "good seals".

There is no garter spring on a wheel cylinder seal/cup. The only thing preventing air entry past the cup lip is the radial force supplied by the cup design/material, and the fluid pressure forcing the cup lip outward.

Newer cups/seals perhaps have more radial force/more efficient seal lips than earlier cups?

Some people are receiving MCs with no RPVs. Why? Ask the vendor, they're supposed to know what they are. A vendor should supply you with the proper MC if you state it's a disc/drum system. RPVs come in an MC rebuild kit but you should not have to install them yourself if the vendor has a clue.

More information is in this thread http://www.camaros.org/forum/index.php?topic=2938.0 Other correct info on brake components can be found at CRG also. The vendors refer to parts incorrectly. Pay attention to what John Z writes.

John Z is wrong.

Photos/scans coming as soon as I can process and upload them. Please give me a few minutes.

 

Interesting.....

My "complete" front disc conversion kit did not come with an RPV for the rear drums. Yet, when you visit the manufacturer's website, they recommend the use of an add-on RPV just aft of the proportioning valve on the rear brake line especially IF THE MASTER CYLINDER IS INSTALLED LOWER THAN THE BRAKE CYLINDERS.

I haven't installed and operated the conversion yet so I have no clue as to the importance of the RPV.

 

Interesting.....

My "complete" front disc conversion kit did not come with an RPV for the rear drums. Yet, when you visit the manufacturer's website, they recommend the use of an add-on RPV just aft of the proportioning valve on the rear brake line especially IF THE MASTER CYLINDER IS INSTALLED LOWER THAN THE BRAKE CYLINDERS.

I haven't installed and operated the conversion yet so I have no clue as to the importance of the RPV.

An RPV is supposed to be in the port for the drums.

I've used residual check valves when I built street rods with the master cylinder under the floor. I've never used one on a setup with a firewall mounted MC. When the MC is under the floor, brake fluid bleeds back into the mc and the wheel cylinders and calipers require more travel for the first stop after a few minutes of driving. I don't remember the numbers, but seems like I used a 6lb on the back and a 2lb on the front. I learned to solve all those problems by mounting the master cylinder on the firewall, where it was designed to be. Gravity then keeps the lines full of fluid and enough pressure to fill the wheel cylinders and calipers. Once I made that change, I always had a brake pedal at the top of travel in both street rods and have never had a problem with any other car I've built. Much simpler system and no ad on's to correct flaws in the basic design of the brake system.

Did you or did you not use an RPV when the disc/drum MC is in the stock location on these cars? If not, explain why they are there and why you think they are not necessary. If I am understanding your statement correctly. Did you look at the photo of an RPV in the link I posted? Gravity has nothing to do with it's operation.

 

I've used residual check valves when I built street rods with the master cylinder under the floor. I've never used one on a setup with a firewall mounted MC. When the MC is under the floor, brake fluid bleeds back into the mc and the wheel cylinders and calipers require more travel for the first stop after a few minutes of driving. I don't remember the numbers, but seems like I used a 6lb on the back and a 2lb on the front. I learned to solve all those problems by mounting the master cylinder on the firewall, where it was designed to be. Gravity then keeps the lines full of fluid and enough pressure to fill the wheel cylinders and calipers. Once I made that change, I always had a brake pedal at the top of travel in both street rods and have never had a problem with any other car I've built. Much simpler system and no ad on's to correct flaws in the basic design of the brake system.

 

Section 2-7: Note that fast release of brake pedal can cause momentary vacuum in the system. This is a situation that can encourage air entry at the wheel cylinders.

(Note at end of) Section 2-9: RPV "Check Valve" eliminated in '70 model year.

 

It has been suggested on another thread that the RPV "check valve" was transferred to the proportioning valve/combination valve.

The Delco manual does not specifically support this belief. I can NOT find evidence that the later systems had an RPV ANYWHERE in the system.

 

It has been suggested on another thread that the RPV "check valve" was transferred to the proportioning valve/combination valve.

The Delco manual does not specifically support this belief. I can NOT find evidence that the later systems had an RPV ANYWHERE in the system.

What thread? I would not think an RPV would be moved from an MC to a combination valve.

 

Really good authoritative post. Thanks loads. :thumbsup: :beers:

 

It's starting to hurt my head to read all this!!

Scott, I used inline residual check valves when I built street rods with the MC under the floor, the MC basically at or below the level of the rear wheel cylinders and front calipers, these are disc/drum combos. I think the pressure listing was different for the drum or disc lines. These valves held the fluid in the line and kept them from siphoning/draining back into the MC. I know without the valves the pedal was near the floor after not using the brakes for awhile.
With the check valves, the pedal remained near the the top and evidently the lines remained full of fluid and also the wheels cylinders and calipers. These are separate/inline valves. Bought from the Ray Godman brake company and were installed on his advice from discussions at a seminar on brakes at the Street Rod Nats many years ago. If there are any check valves in my newer rides, they are integral to the MC as there is nothing I installed in them.

Whatever pressure they hold, I can't imagine they overcome drum brake return springs. Only that they can keep stop any siphoning effect when the brakes are released. I could be wrong.

 

There is a misunderstanding here:

Quote:
Originally Posted by Raven1
I also never stated the RPV was used to adjust shoes.

Shurkey: "You stated:"

Quote:
Originally Posted by Raven1
An RPV maintains 8-16 psi in the rear drum system to overcome the shoe pull-back springs and allows the shoes to stay out close to the drums for instant response to fluid pressure.

Shurkey: "What is that if it isn't adjusting the shoe-to-drum clearance?"

I was talking about physically adjusting the shoes when you install them, not using an RPV for adjustment.

 

It's possible in that link I posted John merely misstated his info and nobody caught it. It happens. I find it hard to believe with his background he's clueless.

Matt, Got ya.

 

When I converted to front discs a while back I bought a master cylinder form the parts store for a 71 Chevelle with disc brakes. I could not get the system to work right until I added a 5lb?? blue?? residual valve in the rear line. After that everything worked fine.

 

WHEEL CYLINDER CUP EXPANDERS ... cheaper and more reliable than rpv's ....this is why Chevrolet stopped putting them in after '69 , right ?

 

Here's my theory on the subject:

First, there was the use of the straight spring:

This is the type of the wheel cylinder that needs a RPV, because when the pressure is released, the spring acts only on the center portion of the sealing cup, hence the cup's edges tends to contract to the center and either fluid will leak or air gets in the system.

Then some smart people thought of adding cup expanders (I guess in 1970) to distribute the force along the edges of the cup instead of the center:

Even smarter people simply changed the shape of the spring ends:

As for RPV, they are still needed if the m/c is below the calipers or wheel cylinders (2 psi) to prevent the fluid to go back to the m/c and causing excessive retraction of the pistons leading to long pedal travel.

As for combination valves with integrated RPV, they do exist, but I don't think they were OEM. Here is the one from CPP (see function #2) and the instructions to convert a disc drum combination valve to a disc disc combination valve (i.e. remove the 10 psi RPV).