power brakes and dual master cylinder plumbing

[drummin89]

I'm preparing to install a firewall mounted pedal assembly with a power booster and dual master cylinder in my 52. I got the booster and master cylinder out of a 99-02 Chevy Silverado (base model truck) and bought the pedal assembly (pirate jacks) from Carolina Classic Trucks.

Anyways I read on their website that when running drum brakes (all around) you need a 10lb residual pressure valve on the line going to the front and going to the back. I also read that its best to put them as close as possible to the master cylinder. I'm planning to follow a tech-tip on the stovebolt about installing a dual master cylinder as far as line sizes, But because I'm mounting the m/c on the firewall and can't leave well enough alone, im going to flip the line to the rear from the passenger side of the frame, to the drivers. All else will be about the same.

My question in all of this is about the residual valves. The residual valves have a 1/8" inlet and outlet, so I imagine I just use a reducer fitting to go from 1/4" down to 1/8"? So the line coming out of the m/c will be 1/4", which then goes to a reducer fitting that goes from 1/4" to 1/8" into the residual valve, then out to another reducer fitting that brings the line from 1/8" back to 1/4" which then goes a tee that splits the line down to 2 3/16" lines to the rear or front drums.

Correct?

I was also told I do not need the 10lb residual pressure valves because the master cylinder is above the brake cylinders. I thought that only pertained to a disc brake setup where the m/c was on the frame and the fluid could siphon back to the m/c if it was almost level with or lower than the calipers.

this is the tech-tip on the stovebolt
http://www.stovebolt.com/techtips/ma...der/index.html

[Houston54]

I see nothing wrong with having the rear axle brake plumbing going down the drivers side as that just makes sense to me. That is how the 52 originally had it.

I am using a swing pedal and brake booster set up from a 79 Grad Prix on my 54 with disc up front and a 10 bolt at the back. I am not using residual valves. I am using a proportioning valve/block from a later model GM pickup. Everything is working just fine.

The residual valves are needed only if you have the MC/booster under the floor in my understanding. I cannot see where they would hurt anything to have in the system but just extra plumbing with no benefit.

The brake booster you are planning to use is a monster. It will take up some firewall real estate and things are tight enough in there as it is. I would suggest using a smaller diameter unit. It would still be compatible with your swing pedal assy. The G-body (79-88 GM montes, cutlass, regal, grand prix, etc) use a seven inch unit that provides plenty of vacumm for the system.

[solidaxel]

I was under the impression that the 10# RV in the rear was to keep the large springs on the brake shoes from completely compressing, henc 10# of residuel pressure on the shoes so you do not have to pump the brakes to take up that first bit of tension on the springs.

[drummin89]

thats what ive been reading too, the job of the residual valve in a drum brake system is a bit different than the job in a disc brake system.

A 2lb residual pressure valve (rpv) in a disc brake system is needed (or recommended) when the master cylinder is almost level or lower than the calipers, (ie frame mounted master cylinder), which could siphon the brake fluid out of the caliper and possibly pull air in.

A 10lb rpv in a drum brake system is needed (or recommended) to counteract the spring tension in the drum which pulls the shoes back away from the drum when the pedal is released, so a rpv keeps pressure in the line, enough to keep the shoes close to the drum so you don't have to pump the pedal a few times to stop.

All I am wondering is, is that how your suppose to plumb a rpv. It just seems weird to go from 1/4" down to 1/8" and then back up to 1/4"

Im only going to be running powered drums for now, eventually switching out the drums on the straight axle to discs, but that won't be for a while.

As far as the size of the booster, im hoping it will be fine, my truck doesn't have a v8 as Im running the 54 235 inline 6 that was in it when I bought it, just with a few extra goodies (tube headers, aluminum intake w/ 4bbl 390cfm holley carb and h.e.i.). The only thing im sure of is that I will have to put on a smaller diameter air cleaner (currently has about a 14in diameter air cleaner).

[Bob@ProAuto]

All of the aftermarket residual valve that I've seen are 1/8" Female NPT. Typically, they include a brass adapter that is 1/8" Male NPT (National Pipe Thread) to 3/8" Female SAE (45 degree flare). This would mean that a 3/16" Flare nut would screw into it. I've never had the need for a 1/4" flare nut, but I suppose that you can get the adapter with 7/16" Female SAE.

As far as the residual valve, these are typically used when the bleeder screws in the calipers or cylinders are lower than the fluid level in the master cylinder. 10# for drum brake and 2# for disc brakes.

If you had the master cylinder under the floor, I would say that you need them or a remote fill reservoir.

I'm not familiar with the booster that you're using, but most boosters require 15-18 lbs. of vacuum. If your motor doesn't create that amount, there are electric vacuum pumps that you could add in.

If you plan on changing to disc brake later on, make sure that the master cylinder you purchase now has the fluid capacity for disc brakes. Calipers need more fluid than wheel cylinder. Some master cylinders, even with "equal sized" fluid reservoirs, have a specific port for the front brakes and one for the rear.

You didn't mention an adjustable proportioning valve. I would recommend this. I use the one that ties into the rear brake line so that the pressure can be adjusted. You want the front brakes to grab just before the rear ones. Otherwise, you may swing the rear around......

And yes, you can run the rear brake lines on the driver's side.

[drummin89]

this is where I was getting some of my information

http://www.piratejack.net/Tech-Suppo...gurations.html

[Bob@ProAuto]

I've never used a residual valve when it's been a firewall mounted master cylinder.

[drummin89]

OK, well the general consensus I've been getting is to do away with the residual pressure valves so thats what im going to do. I am going to plumb in a adjustable porportioning valve for the rear. Where would the best place to mount this be? Is it possible or safe to mount it in the cab (say under the dash or on the floor by the drivers seat)? Or is it best to just keep it out of the cab on the firewall or frame?

So nothing goes on the line to the front? Just a 1/4" line coming out of the master cylinder, which will go to a Tee mounted close by the driver front wheel. Coming out of the Tee will be two 1/4" lines, one looping around to the driver front wheel, and the other running along the front crossmember to the passenger front wheel.

The rear will have a 1/4" line coming out of the m/c which will go into a adjustable porportioning valve mounted somewhere via a reducer (1/4" down to 1/8"). Out of the apv the line will jump back up to 1/4 via another reducer (1/8" up to 1/4") and from there back to a Tee mounted on the rear axle. Out of the tee will be 2 3/16" lines going to the wheel cylinders. Correct?

[Bob@ProAuto]

Before I reply, I'm going to be straight up with you. I have a "truck related" business. I try to throw out information if I can keep someone out of trouble.

I've taken the time to send a message to the "Staff" here and see what I need to do to be a Site Vendor, before I get myself in trouble. Once they get back to me, and if it is reasonable, I'll throw out my opinions to you, along with some photos on how we lay-them out.

I've never been black balled or banished from a Forum, so I'm not going to start now.

[drummin89]

ok, once you find out, either PM me or email me at rockfan1815@yahoo.com

Im sure they wouldn't mind you helping me out with descriptions, diagrams and photos as long as you are not advertising your business or trying to sell me a product on here without becoming a vendor.

[drummin89]

Here's another thought/question (I know Im full of them). Would you be able to use a combination valve (porportioning valve/metering valve/differential pressure switch) with a drum brake system. It seems logical that I could, seeing that the porportioning valve would regulate the rear pressure, the metering valve would hold off the fronts until the rear start braking (I know this is only for cars with front disc's) and the pressure switch would just be an extra as I could mount a warning light in the cab.

Im planning on putting disc brakes on the front so this will have to be plumbed in eventually. And it will only be in this configuration driving around my yard as it will have front disc's by time it hits the road.

What do you think?

[mr48chev]

Well, I can tell you that I once had a big firewall mounted mastercylinder and booster on the 48 and along with being big and ugly hanging there it got in the way of anything you wanted to do on the left side of the engine. That was when I had a 67 Camaro subframe under the truck and disk brakes on the front.

Currently with a stock frame and I beam I have a 68 C-10 dual mc on a custom bracket mounted to the stock location with a custom pushrod. This worked pretty good for close to 15 years and 100K miles with 54 passenger brakes on the front and 72 Nova brakes on the rear.

Current plans are for four wheel disk with a booster an mastercylinder combo mounted under the floorboards on the frame.

From all the info I have accumulated in the past 40 years of working with disk brakes the 2lb residual valve for the disks is correct and the higher rated one is correct for drum.