residual pressure valve - Team Camaro Tech
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post #1 of 14 (permalink) Old Jun 18th, 08, 03:37 PM Thread Starter
David
 
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residual pressure valve

Hello all,

I converted my 68 with drum drum to power disc/drum setup with 11" discs and single piston oem type calipers and brass combination valve. The kit did not come with a residual pressure valve in the master cylinder or one that mounts in line. The MC is mounted on the booster in the stock location and I want to know if I need one. I have a firm pedal but the pedal goes down about 2 inches before the car begings to stop. I adjusted the shoes out and made sure the booster pin is around 1/16" from touching the MC and power bled the system until no air bubbles. Adjusting the clevis just masks the problem. The pedal is firm after around 3.5 inches of travel or more. I have read that power brakes only require around 1.75 inches to operate. I have been running like this for a few years and want to fix it. I have seen conflicting info on brake TS sites saying I need one and the tech rep said I probably don't. Anyone with a conversion please tell me if the valve will solve my problem. Thanks.
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post #2 of 14 (permalink) Old Jun 18th, 08, 03:50 PM
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Bob
 
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Re: residual pressure valve

You do not need a residual valve. The combo valve handles distribution and proportioning.

The booster pin-master gap sounds too large. Should be about 10 thousandths, not 1/16 inch. That gap is multiplied by the time you get to the end of the brake pedal. My guess is this is causing a lower than normal pedal.
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post #3 of 14 (permalink) Old Jun 18th, 08, 05:40 PM
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Re: residual pressure valve

I don't agree. ANY drum system requires a residual pressure valve, or you'll get exactly the symptom described - excessive pedal travel because so much fluid is displaced moving the drum shoes out from rest against the pull-back spring pressure to get them to the drum surface. This has nothing to do with distribution or proportioning.

The residual pressure valve maintains 10 psi in the drum fluid system, which keeps the shoes out close to the drums to minimize pedal travel on brake application. Drum/drum systems use one in each outlet port, disc/drum systems use one only in the rear outlet port that serves the drums, and disc/disc systems don't use any.

The photo below shows a drum/drum master cylinder, with RPV's in both outlets; item 1 is the RPV spring, item 2 is the RPV, and item 3 is the brass tube seat.

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post #4 of 14 (permalink) Old Jun 18th, 08, 05:56 PM
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Re: residual pressure valve

I agree with John. But I have a question for you John. One time I got to thinking about this and couldn't keep from returning to this thought.... the MC has 2 pistons one of which goes to the front discs which require no RPV as the discs float close to the rotor.... so if the rear has no RPV and the shoes have returned to the stops.... when you push the break pedal you still move the front discs against the rotor even though the rear may be nowhere near the drum. Shouldn't the symptom of no RPV on a disc/drum be that the car wants to do a somersault on a hard braking instead of a soft pedal? Since there is only one piston rod with 2 pistons on it I have trouble with what causes the soft pedal unless we are saying that the MC is thereby able to deliver all its force to just the front, and the additional travel is indicative of how much work the rears would do if they were helping. And that makes sense too.

Tim Smith
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post #5 of 14 (permalink) Old Jun 18th, 08, 06:00 PM
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Re: residual pressure valve

Quote:
Originally Posted by JohnZ View Post
I don't agree. ANY drum system requires a residual pressure valve, or you'll get exactly the symptom described - excessive pedal travel because so much fluid is displaced moving the drum shoes out from rest against the pull-back spring pressure to get them to the drum surface. This has nothing to do with distribution or proportioning.

The residual pressure valve maintains 10 psi in the drum fluid system, which keeps the shoes out close to the drums to minimize pedal travel on brake application. Drum/drum systems use one in each outlet port, disc/drum systems use one only in the rear outlet port that serves the drums, and disc/disc systems don't use any.

The photo below shows a drum/drum master cylinder, with RPV's in both outlets; item 1 is the RPV spring, item 2 is the RPV, and item 3 is the brass tube seat.


John,

You are absolutely correct about an all drum system,however the car was converted to front disc brakes. The disc kits come with combo valves,which regulate/maintain the pressure for front disc/rear drum setups. The only reason he should need a resid valve is if the master was located below the caliper's level. Pressure is maintained through the valve.

Here is the complete troubleshooting chart for "low pedal". Drum brakes and the use of a resid valve are also mentioned. I'm banking on a bad master, or bad booster pin to master clearance, or maybe still some air in the system somewhere. He should not need to use a resid valve if he has a combo valve installed. Pedal travel should be fine out of the box. Once it is set up properly,a resid valve would be a great way to result in a firmer pedal,and faster reacting rear drums.- A nice enhancement for sure. Right now I think it might just be masking the real problem though.


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post #6 of 14 (permalink) Old Jun 19th, 08, 08:16 AM
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Re: residual pressure valve

As John said, the drum (rears I assume) needs a RPV. You can get the Wilwood 10lb (part #950-260-3279) and put it in line. The Disc use a 2lb, but many say this isn't needed. Also, as Bob said, pull the MC and check the depth of hole and the length of the push rod. It should be close if it is the wrong one, i.e. if the long hole it will be about 1 1/2" short will be 1/2" and the rod should be proportional to the hole depth.
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post #7 of 14 (permalink) Old Jun 20th, 08, 08:22 AM Thread Starter
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Re: residual pressure valve

Thanks for the input. I will get the valve and if the drums drag, I can always pull it off. Now, I need to learn how to flare the lines...
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post #8 of 14 (permalink) Old Jun 20th, 08, 05:45 PM
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Re: residual pressure valve

Quote:
Originally Posted by honers camaro View Post
Now, I need to learn how to flare the lines...
You will require a double flare,nothing less.You will require a qaulity tool here to do the job correctly.Snap On,Mac tools etc.No corner hardware or big box store flare kit junk for aluminum or copper tubing.

George
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post #9 of 14 (permalink) Old Jun 20th, 08, 06:52 PM
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Re: residual pressure valve

Quote:
Originally Posted by 77wolf10.85 View Post
I agree with John. But I have a question for you John. One time I got to thinking about this and couldn't keep from returning to this thought.... the MC has 2 pistons one of which goes to the front discs which require no RPV as the discs float close to the rotor.... so if the rear has no RPV and the shoes have returned to the stops.... when you push the break pedal you still move the front discs against the rotor even though the rear may be nowhere near the drum. Shouldn't the symptom of no RPV on a disc/drum be that the car wants to do a somersault on a hard braking instead of a soft pedal? Since there is only one piston rod with 2 pistons on it I have trouble with what causes the soft pedal unless we are saying that the MC is thereby able to deliver all its force to just the front, and the additional travel is indicative of how much work the rears would do if they were helping. And that makes sense too.
If there's no RPV in the rear outlet of a disc/drum master cylinder, you'll definitely get a case of "front brakes first" (nosedive) until the rear shoes expand enough to meet the drums, especially on light brake application at lower speeds.

There are two separate pistons in the master cylinder bore, not one with two sets of seals on it.


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post #10 of 14 (permalink) Old Jun 20th, 08, 07:07 PM
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Re: residual pressure valve

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Originally Posted by BelAirBob View Post
The disc kits come with combo valves,which regulate/maintain the pressure for front disc/rear drum setups.
The combination valve (typical photo and cross-section below) has nothing to do with the residual pressure function on a disc/drum system; the combination valve handles distribution, the differential pressure warning switch, hold-off metering, and front/rear high-pressure proportioning, but the RPV function is entirely in the master cylinder outlet (in OEM master cylinders). With aftermarket master cylinders (like Wilwood, etc.), the drum side of a disc/drum system needs an external RPV.

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post #11 of 14 (permalink) Old Jun 20th, 08, 08:25 PM
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Re: residual pressure valve

Quote:
Originally Posted by JohnZ View Post
The combination valve (typical photo and cross-section below) has nothing to do with the residual pressure function on a disc/drum system; the combination valve handles distribution, the differential pressure warning switch, hold-off metering, and front/rear high-pressure proportioning, but the RPV function is entirely in the master cylinder outlet (in OEM master cylinders). With aftermarket master cylinders (like Wilwood, etc.), the drum side of a disc/drum system needs an external RPV.

Thanks for the diagrams... So... it sounds like the "OE style" masters might already have the RPV then? The reason I ask is that we've never had to use external RPVs with our disc brake conversions. We do not use the Wilwood masters. Ours look like OE units. Have not looked closely at the ports though to look for the RPVs. Just curious.
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post #12 of 14 (permalink) Old Jun 22nd, 08, 06:00 PM
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Re: residual pressure valve

Quote:
Originally Posted by BelAirBob View Post
Thanks for the diagrams... So... it sounds like the "OE style" masters might already have the RPV then? The reason I ask is that we've never had to use external RPVs with our disc brake conversions. We do not use the Wilwood masters. Ours look like OE units. Have not looked closely at the ports though to look for the RPVs. Just curious.
Bob, a correctly-configured "OE-style" master cylinder for a disc/drum system will have an internal RPV in the rear outlet port, behind the brass tube seat; if you probe gently through the hole in the tube seat with the blunt end of a drill bit, you'll feel the rubber valve. A disc/disc master won't have any.


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post #13 of 14 (permalink) Old Jun 22nd, 08, 06:41 PM
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Re: residual pressure valve

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Originally Posted by JohnZ View Post
Bob, a correctly-configured "OE-style" master cylinder for a disc/drum system will have an internal RPV in the rear outlet port, behind the brass tube seat; if you probe gently through the hole in the tube seat with the blunt end of a drill bit, you'll feel the rubber valve. A disc/disc master won't have any.

Ok, that explains it then. We use the OE style,which is a GM replica. Thanks for the feedback.
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post #14 of 14 (permalink) Old Jun 23rd, 08, 05:58 AM Thread Starter
David
 
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Re: residual pressure valve

OK, I was thinking that it may be a lot easier to look into getting a new master with the valve behind the seat. The one I have now is this
http://www.mpbrakes.com/uploads/prod...full_large.jpg
which looks like a 60s corvette style. The bore on mine is 1". Can I get one from my local parts house that looks physically the same but has the valve in it or does anyone have the same looking unit with a part nuber I could use? Thanks
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