The Stovebolt Site Forums The Shop Area Driveline Brake question!

I have a 70 GMC half ton with 6 lug 4 wheel drum. I would like to add a power booster to make stopping easier. The question is If I add a power booster and corvette style master cylinder how to I plumb in a adjustable proportioning valve? I was told by the supplier to add it in the line that goes to the rear brakes. Makes sense but if its a dual master cylinder, how does it proportion the fluid? Also do i need to add the residual pressure valves to keep 10lbs of pressure in the lines? My head hurts!!

Thanks,
Mac

Hy handymanmac, I think that all you would need to do is install the power booster in front of your existing master cylinder. Corvettes had disc all around, hence no residual pressure valves built into their master cylinders, so they must be added when using with drum brakes. Do you currently have a problem with your rear brakes locking up when stopping? If not why do you feel an adjustable proportioning valve is required?

I would not use a Corvette MC for this application. As 3B stated you may be able use the stock dual MC as it has built in residual valves. Check your local parts store to see if the MC is different for non PB vs factory PB. If it is use the stock MC that is used with a booster. In most cases a proportioning valve is not required with drum brakes.
Fred

Thanks for the responses. I bought my current set up from a supplier that came with a booster, MC and Prop valve. It stinks. The rear's lock up way before the front. Then I question the prop valve. It should be a distribution block and not a prop valve, right? So I figured I could remove the prop valve and add an adjustable valve to control the rears. Its frustrating that each supplier I talk to has a different idea but they all seem to say to use a manual prop valve and loose the prop valve at the MC. Then I can add the residual valves in line to provide the residual pressure that the corvette MC lacks. Make sens?!

Hy handymanmac, it makes sense, especially if you're selling new parts. So you're currently experiencing rear wheel lockup when braking?

Here's some more info. I did a complete brake rebuild front and rear. Breaking was fine but just harder on the old leg. I added power steering and while I was at it, I figured i'd use a booster. Installed the set up and had poor braking response. When I did stand on the brakes the rear's would lock well before the front. overall I had very soft brakes.

Right now I have the engine out and front clip off I'm repainting the firewall and front frame rails and have disassembled the a-arms to be repainted. while everything is apart, I figured its easier now to work on the brakes and plumbing.

Hy handymanmac, basically an all drum system doesn't need a proportioning valve, the reason you're getting rear brake lock up is because of weight transfer, what little weight there is in the rear of a pickup. I would rather see you use a weight adjusting proportioning valve, the type that the manfacturers use to vary the amount of rear braking effort based on the weight at the back of the truck. Once you "dial in" an adjustable proportioning valve, that is the maximum braking effort you will get in the rear reguardless of load, hope that helps,

a $40 adjustable proportioning valve is the way to go.

I am going to stick my nose in here. I am not being critical here but this is my observation.

Why would someone buy after market kits for a truck that came stock with these parts. A 1970 truck is not that old, that most parts are not available at your local FLAPS. The other thing is there are still lots of these trucks in wreaking yards, even here in busy SOCAL, for brackets and such.

I do not remember if a 70 had a dual system, but if it didn't it wasn't long before GM had a dual system in a truck. In this case you would just buy the first year parts. They will bolt right up without much trouble.

Go to your local parts store and buy the stock parts over the counter. MC, Booster and stock P. valve.

Just my take on this situation.

Now if you had a 54 like I do, then you start assmebling the after market parts and reading lots of article and online tech article from the kit manufacturers.

I hope you get your braking problem worked out. To educate youself, go to the brake kit vendors; they will have tech articals to answer all of your questions.

That's exactly what I was thinking. The '70 would have a dual system from the factory, so that isn't an issue. Personally, I would look for OE replacement parts.

Here's an update! I went to my parts guy who still knows how to
look things up in a catalog, and I found that the orginal MC was the same for both power and man brakes. So adding the booster shouldnt make a difference in braking. However, he looked up the rebuild kit for the brakes which includes the wheel cylinders and the part #'s are the same for front and rear. Jamie the parts guy, picked up on that being wrong! The front and rear wheel cylinders are different diameters. 1 1/8" front and 7/8" rear (I think). So when I rebuilt the brakes I probably have the wrong wheel cylinders.

As for junkyards here in NJ there are little to know old parts or trucks. When scrap was sky high the cars were sold off. Also the after market set up is cheaper than buying stock. At least at my store.

I'm going to look at the wheel cylinders this weekend and check. I'll keep you posted.

Mac

Hy handymanmac, my Raybestos catalogue shows 1 1/8" front p/n 37025 left, p/n 37026 right, rear 1" p/n 13387 left, p/n 13388 right, there is also a listing for heavy duty brakes, same bore sizes but different front wheel cylinder p/ns 19086 left. 19087 right, hope that helps.

Thanks 3B

I am going to take a look at the cylinders tomorrow. My thought is the rears are wrong. I'll keep you posted.

Mac

Here are the findings! The rear wheel cylinders are the same as the front. They should be 1" and are 1 1/8 . I ordered new ones from raybestos for both front and rear. I also ordered a new M/C and a rebuilt. We looked at both and decided to go with the rebuilt as the new ones are made over seas and the casting was horrible. The new cost $70.00 while the rebuilt was $17.00 with no cre charge. lastly the Corvette style M/C has no residual pressure so that also added considerably to the issue. We also ordered in a booster but found that it does not have the mounting brackets so I will stick with the MPP one I have. At least now I wont have to throw an anchor out to stop.

Thanks for everyones help!! Mac