Residual Pressure Valves (RPV's)...the more I read, the less I know [Archive] - Chevelle Tech

: Residual Pressure Valves (RPV's)...the more I read, the less I know


69 chevelle 25 years
May 15th, 12, 9:53 PM
Okay, residual pressure valves contradictions I've read ...

1. I've heard, - rpv's are necessary with drum brakes ... but, ..."most replacement master cylinders don't have them" so, why are they in original master cylinders but not in replacements ??
2. I've heard, - rpv's hold the brake shoes out for more immediate braking response ... but ... I've also heard that they don't hold enough pressure to overcome the return springs (this makes sense to me because old time racers would switch out their discs for drum brakes to reduce drag) ...
3. I've seen posts where it is mentioned that someone's new master cylinder isn't even machined to be able to fit a "RPV" inside ... how can that be if they're necessary on drum brakes ??
4. I've seen posts and tech articles say that the purpose of the residual pressure valve is not to hold out the brake shoes, nor to make their response more immediate , ...but, it's to keep air from entering the wheel cylinders ... this also makes no sense to me ...can it be true that air will pull itself back into the wheel cylinders without the residual pressure ???
I have an "el-cheapo" parts store rebuilt "4-drum" master cylinder from 1993 lying around ... can i tear into it to look for residual pressure valves? ... if I find them, can I pull the valves out to use in a new master cylinder's drum brake circuit?? ...


what are the real answers to these mysterious questions ???!!!

Raven1
May 16th, 12, 6:29 AM
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.

dream66
May 16th, 12, 8:33 AM
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.

oktunes
May 16th, 12, 9:04 AM
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.

Raven1
May 16th, 12, 9:29 AM
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.

Schurkey
May 16th, 12, 2:36 PM
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.

Schurkey
May 16th, 12, 3:13 PM
http://hbassociates.us/Delco_Brake_Manual_01.jpg



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.

http://hbassociates.us/Delco_Brake_Manual_02.jpg


(Note at end of) Section 2-9: RPV "Check Valve" eliminated in '70 model year.
http://hbassociates.us/Delco_Brake_Manual_03.jpg

Schurkey
May 16th, 12, 3:25 PM
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.

dream66
May 16th, 12, 3:31 PM
Really good authoritative post. Thanks loads. :thumbsup: :beers:

Raven1
May 16th, 12, 4:02 PM
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.




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?



John Z is wrong.

Photos/scans coming as soon as I can process and upload them. Please give me a few minutes.
I know what a hold-off valve does. I never mentioned it's operation. I also never stated the RPV was used to adjust shoes.

If there is line pressure of 8-10 PSI between the check valve and brake wheel cylinder cups, isn't that pressure in opposition to spring pressure? I also did not think air was supposed to enter a wheel cylinder if the components were adjusted properly and in good condition.

I was under the impression his information was correct due to his background. Perhaps any info posted should be regarded as 100% false unless completely validated by numerous sources. I generally never believe anything I read. In John's case I assumed it was correct. It would seem best to doubt everyone as a precautionary measure. Thanks for posting the copy of the manual.

Raven1
May 16th, 12, 4:05 PM
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.

Schurkey
May 16th, 12, 4:52 PM
I know what a hold-off valve does. I never mentioned it's operation.
Correct. I mentioned it as further proof that the RPV doesn't keep the shoes near the drums.

I also never stated the RPV was used to adjust shoes.
You stated:
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.
What is that if it isn't adjusting the shoe-to-drum clearance?

If there is line pressure of 8-10 PSI between the check valve and brake wheel cylinder cups, isn't that pressure in opposition to spring pressure?
Yes, it opposes spring pressure (tension). You therefore have a situation where ~16 psi opposes ~50+ psi. Which results in the shoes coming to rest on the anchor pin unless there's something wrong with the brake mechanism such as an over-tensioned or seized park-brake cable; or the shoes are hung-up on the grooves worn into the brake backing-plate.

I also did not think air was supposed to enter a wheel cylinder if the components were adjusted properly and in good condition.
Yes. Air "shouldn't" enter the system. Apparently, with the master under the floor (siphon effect-fluid over-fills the reservoir) or on the firewall but with marginal wheel cylinder cups, air DOES get into the system past the wheel cylinder seals. This might be made worse if the "venting" described in the Delco manual--where fluid from the reservoir bypasses the master cylinder cup(s) under momentary vacuum--isn't effective; perhaps because it's -30 below and the rubber cups are stiff and the fluid is cold and thick.

I was under the impression his information was correct due to his background. Perhaps any info posted should be regarded as 100% false unless completely validated by numerous sources. I generally never believe anything I read. In John's case I assumed it was correct. It would seem best to doubt everyone as a precautionary measure. Thanks for posting the copy of the manual.
Life's good. We're OK. Happy to put this issue to bed.

What thread? I would not think an RPV would be moved from an MC to a combination valve.
Doing a quick search. I read about RPVs in the combo valve fairly recently, but this thread is many years old. It's not the one I remember. Post #8.
http://www.chevelles.com/forums/showthread.php?t=144867

Even older--Post #2
http://www.chevelles.com/forums/showthread.php?t=29739

From 2010--Post #3
http://www.chevelles.com/forums/showthread.php?t=303465

None of these posts is the one I "sort of" remember reading recently--but--they all claim that there's an RPV somewhere else in the brake system.

I'll continue to look...

[Edit 1]
This is interesting. Post 2--cup expanders. Completely forgot about the spring windings!
http://www.chevelles.com/forums/showpost.php?p=3451783&postcount=6

[Edit 2] I give up. I can't find a recent post about RPVs in the combo valve. Maybe it was on another forum.

oktunes
May 16th, 12, 5:59 PM
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.

Raven1
May 16th, 12, 7:40 PM
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.

Raven1
May 16th, 12, 7:44 PM
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.

69 chevelle 25 years
May 16th, 12, 11:57 PM
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.


WOW - thanks so much, Schurkey ... that delco information is great stuff... I no longer have a fear of installing my new master cylinder and worrying about RPV's ... if Chevrolet didn't need them in 1970, I don't care if I have one now ... additionally, I swear I, too, can remember seeing a thread somewhere stating that the RPV was transferred to the combination valve...and I, too, can't remember where (no matter) ... thanks again , from the seasonally afflicted wastelands ...

blumont
May 17th, 12, 9:09 AM
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.

69 chevelle 25 years
May 20th, 12, 12:22 AM
WHEEL CYLINDER CUP EXPANDERS ... cheaper and more reliable than rpv's ....this is why Chevrolet stopped putting them in after '69 , right ?

Jack Action
May 20th, 12, 12:15 PM
Here's my theory on the subject:

First, there was the use of the straight spring:

http://www.armyradio.com/publish/MB-GPW_Jeep_Parts/Pictures/pic-57.jpg

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:

http://ww2.justanswer.com/uploads/working4ev/2010-02-10_042329_1.gif

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

http://www.caddydaddy.com/images/products/15788.jpg

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 (http://www.classicperform.com/Instructions/PDF/PVK.pdf) (see function #2) and the instructions to convert a disc drum combination valve to a disc disc combination valve (http://www.classicperform.com/Instructions/PDF/Prop_Valve_Mod_Instructions.pdf) (i.e. remove the 10 psi RPV).