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Old 02-01-2013, 11:36 PM
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Originally Posted by chevyrestore View Post
No when it had disc drum setup I used the MC with an inline residual pressure valve to provide more pressure to the rears only. When I switched to disc disc all I did was remove the inline residual valve and add the disc disc proportioning valve. I called CPP and they said when I had drums it was set up right and now that it has disc disc its set up correctly also since I removed that residual valve and added the proportioning valve (non adjustable from CPP)
Dude, we need to talk. There is some confusion over parts and terminology here that we need to clear up. Don't take offense, there is much confusion regarding brake systems and their various components and what they all do.

First off, the residual pressure valve does not provide more pressure to the rear brakes. What residual pressure check valves do is hold pressure in the lines when your foot is off the pedal. That is why they are called "residual pressure" check valves. 10lbs for drums (always) and 2lbs for discs if your master cylinder is lower in the car than the calipers (keeps the pistons in the calipers from pushing fluid back to the master cylinder and moving the pads away from the rotors). If you have discs on an axle you need a 2lb residual pressure check valve in the line that feeds it (if your master cylinder is below the caliper level). If you have drum brakes you need a 10lb residual pressure check valve in the line that feeds that end of the car (always).

In a disc/drum master cylinder the 10lb residual pressure check valve is in the outlet of the master cylinder that gets plumbed to the drum brake equipped axle. It can be removed (usually) so that same master cylinder can be used for a car with disc/disc.

Aftermarket brake system manufacturers, in an effort to simplify their inventory only stock master cylinders for a disc/disc system. No check valves at all, just like yours. Consequently the check valves need to be added elsewhere in the system if they are required, along with a proportioning valve and a metering valve if they want to include them. They do that most typically by selling you what is known as a "combination valve". That is what you have plumbed near your master cylinder.

This is where several assumptions need to be made by the manufacturer of your brake kit. First off, they need to know if you have disc/drum or disc/disc, because that changes the proportioning and the metering requirements of the brake system. Secondly and most importantly they need to make a guess as to the weight of your car, the center of gravity location, tire size etc. This is where it gets ugly. The OEM manufacturers test each specific car with different combination valve settings until they get balanced braking that is safe for that car. Now, in your truck for example if you take out a small block and add a big block, and then change out the rear tires for monster Mickey Thompsons and then lower the suspension, you have just changed the braking balance for your car, and likely your combination valve is no longer going to provide balanced braking. One size does not fit all when it comes to combination valves.

What the manufacturer described in that instruction sheet was to take out the proportioning part of the combination valve, which will likely increase the line pressure to the rear brakes (because it is no longer proportioned, the port is now wide open to full master cylinder pressure). That will increase the rear braking pressure needed for disc brakes, but it may not give you a balanced brake system any more. Having the rear brakes lock up first is bad, unless you like spinning out when you have to do a panic stop in the rain. That is where I have a problem with what the paper describes. Also in their conversion instructions you still have a 10lb drum brake check valve on a car converted to discs. As I said, that will cause the pads to drag on the rotors.

With all that being said, I know that you have installed on your truck a disc/disc combination valve (according to your research and what the manufacturer stated). It should have proper proportioning and metering for whatever vehicle the parts manufacturer tested it on. It MAY not be correct for your particular truck. This is where your problem could be. Do you think your truck has the same braking characteristics as a Camaro with a disc/disc system for example? I don't think so...Do they sell the same combination valve for trucks and cars? Again, I don't know, but if the choice in their catalog is either disc/disc or disc/drum I would say that two part numbers cover all the kits they sell for every type of make and model. We all know that there are more than two configurations of cars or trucks on the street. That is why I say it is a crap-shoot if it will work okay when hard braking.

What I am trying to do by walking you through all these steps in your braking system is to validate what you have installed and that you have installed it properly. It is very challenging to do without seeing the vehicle.

What I suspect is wrong with your truck assuming all the rubber lines are good (they probably are not causing your problem). If your rubber hoses were failing the 1000 or so psi your are putting through them every time you push the pedal will make that very clear, and assuming that your brake pedal or master cylinder mount isn't flexing, and that the pedal linkage is travelling through the center of its stroke, the problem is probably the combination valve. I just didn't want you spending money to replace it without checking out everything else first.

If you get good flow out of the master cylinder and it is noticeably reduced out of the combination valve, I would look into replacing it. Not with another combination valve, but with a stand-alone 2lb check valve in both the front and rear lines, and an adjustable proportioning valve in the rear line. You could also add a stand-alone metering valve in the front line, but I have rarely found it necessary. After installing all those parts, adjust the proportioning valve for full rear brake pressure, re-bleed the brakes (you are probably pretty good at it by now) and then go for a drive someplace (like the gravel parking lot you need to find) and do some adjusting. I think you will find your brakes much improved.

I also need to mention that I wouldn't trust a Corvette master cylinder (which is what you have) with a 1 1/64" bore. They didn't come that way. I would likely replace it with a new unit that has a 1" bore when putting in the new valves.

I hope this helps to clear up some confusion when we all are talking back and forth about brakes.


As to all the other folks out there who are into this stuff, I know I made several assumptions in my description of aftermarket braking systems, and there is MUCH more detail we could cover. I am trying to get the point across and also be understandable. What I have described is a summary of my findings from years of experience working with OEM, aftermarket and custom brake systems.
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