Proportioning valve with rear discs brakes.

[Vince M]

I've ordered rear disc brakes and was wondering what is the best proportioning valve and master cylinder to use with them. I was thinking that a four wheel drum brake proportioning valve and master cylinder would work. My thinking is that there is not the huge mismatch in pressure if all four wheels are discs. There obviously needs to be more pressure applied to the front vs the rear but a drum brake system would already have this.
Anyone have any thoughts on this pros or cons.

[neonlarry]

I would pick one of the later GM master cylinders and proportioning valves for a rear disc vechicle. With disc brakes you really need the volume and piston size of a disc brake master cylinder. I just read a post a few days ago where someone used vette ones.

[Longhorn Man]

Travis, you were reading my mind on the master choice. Find what the first year was for a 4 wheel disk on a vette, and compair that master cylinder with yours. You need to pay special attention to the spot where the pushrod goes. You want it to be the same depth as your current one. Some are almost flat, while some are concave an inch...that would cause almost no pedal, it would swing freely for the first 3/4 of the way down or so.
As for a proportioning valve, i would install a drum/drum one, and then an adjustable one in the line going to the rear wheels. That way you could fine tune it.
Even after all this, you'll probably be surprised at how little your braking is improved. I did it once on my old 13 second Honda, (yes, i used to be on the dark side) and I felt violated when the results were so minimal...it also slowed me down a 10th in the 1/4 due to the added weight and the added friction.

[Jeepster376]

A vette is very different weight distribution from a pickup. Use your existing master cyl, throw away the factory prop valve and get a wilwood adjustable prop valve. You just run the front brakes straight, then the wilwood valve goes in the rear line. You take it out to a parking lot and lock em up a few times, and just adjust the valve untill the fron and rear lock up together, and your done.

[Longhorn Man]

Jeepster, the trouble with that is, a disc brake master cylinder needs to hold more fluid in reserve than a drum one. If you look at your master, you probably have a mmall side and a big side on yours, since you have a disc/drum on yours.
That doesn't even get to the fact that drums operate on WAY less pressure than discs, and you will hardly get any rear braking on the old master.

[crustysarge]

Hey guys, I thought I would throw this in too. I have been playing around with brakes on my 77 Trans Am after adapting the rear discs from a 98 Trans Am donor car. Much to learn about brakes, believe me. I have been experimenting with different master cylinders and have yet to get it to where I would consider "right". Anyway I pasted the statement below about residual brake pressure because it is VERY important. If you try running a drum set up Prop. Valve on rear discs that ain't too good. Do some research on the net under "residual brake pressure", you will find lots of info. A drum setup will apply 10 PSI which is too much for discs. Now if you decide to use a manual valve and the M/C is higher than the brakes, like the truck in question you should be OK.

Residual pressure valves hold a preset pressure from the valve to the brakes be it disc or drum. On some cars with the master cylinder lower than the brakes on each wheel, by holding a set pressure it insures that the brake fluid does not siphon back into the master cylinder. Also as with the case of drum brakes the return springs installed on the brake shoes will collapse the slave cylinder where you would not have contact from the shoes to the drums with just one push on the pedal. With 10 psi residual pressure held, the shoes are held close to the drums for instant braking on the first push of the pedal.

[raggedjim]

Classic performance products sells a Proportioning valve for $59.00 that is setup to be used on these trucks running an early (1968-1972) corvette master cylinder. I have one on order and will let you know how it works (may be a week or two because of my schedule...)

RG

[Jeepster376]

Originally Posted by Longhorn Man

Jeepster, the trouble with that is, a disc brake master cylinder needs to hold more fluid in reserve than a drum one.

Yes your right. I thought of that after I posted what I did, the disk calipers can require more fluid volume than the drums.

[Longhorn Man]

crustysarge, for your T/A, have you thought about the 81 annv model for a master cylinder? It had 4 wheel discs off the show room floor.

[crustysarge]

Yes I have. 79-81 had them and I currently have a recon 79 with the small booster I bought at NAPA while in the States. I will get around to installing it this summer and see how it goes. Not cheap, paid around $260 for the thing. Boosters are not cheap!!!

[68LSS1]

Keep in mind the car stuff isn't always better than the truck stuff. I recently got my Hydratech hydroboost and had to decide on master cylinders. I figured I'd go with a late model Vette but the bore size was smaller than the truck master cylinders (relationship to weight of vehicle and bore size, or fluid displacement). Most all the GM cars were a inch or under. Late model trucks had some 1 1/8". I found out that a '99 truck master cylinder (4 wheel disc) has a 1 1/4" bore size.

[Jeepster376]

Bigger isn't necessarily always better. A smaller bore diameter will generate a higher mechanical advantage for a given caliper piston diameter, at the cost of fluid displacement. So a smaller MC bore will give you more powerful brakes all else being equal. The only problem is if it's too small it may not move enough volume and you could get a low or mushy pedal.

[LONGHAIR]

The residual pressure is important but that is accomplished in the proportioning valve not the master cylinder itself.
The volume difference if the reservoir is not for actual braking, it is there to compensate for the loss of volume to the calipers. As the pads wear down and the piston stays further out, that volume of fluid doesn't get back to the master cylinder. Since the rear brakes are adjusted by the lower adjuster screw, the adjustment doesn't use more fluid. The springs draw the tops of the shoes back to the same resting place.
Of course this is all for the "average dummy" that just puts in gas and drives.
You should be able to use the stock master cylinder and an adjustable proportioning valve. Just keep an eye on the fluid level as the brake pads wear.
As some of the others have said, don't expect a great difference in stopping power though. Drum brakes stop very well. The big reason for discs on the front is the "fluid adjustment" that keeps the braking pressure more equal. This keeps the truck from pulling to one side when the brakes are applied.
Disc brakes are far easier to keep clean, as far as mud and yuk goes on a 4X4, nothing gets trapped in the drums. You lose braking power when they are wet though.
If you are doing this for "looks", it can be accomplished, just don't expect much better performance.

[Longhorn Man]

Woah, I think you are the first to agree with me about the minimal gain in performance with rear discs.
Cool.

[68LSS1]

Even going with aftermarket big brakes like I'm doing, probably won't decrease stopping distances by any. Tires are the best brake upgrade (next to a hydroboost unit anyway ).

[Vince M]

Thanks for all the input on the subject. I guess I know which way I'm going now. I have access to a few different master cylinders and I'm going to try different ones to see which works better. I'm probably going to get an adjustable prop valve and play with it. Front lines are going to go direct and the prop valve out back.
How many people have removed their drum brakes to find the axle seals leaking and oil collecting on the inside of the drum. In this case rear drums don't work at all. I still have to disagree that discs don't provide some advantage.

Thanks again for the input..

[Longhorn Man]

They DO decrease stopping distance, but not very much at all. Naturally though, there may be that one time where 3 feet is the difference between crapping your self, and bending your hood over a honda.
Leaking wheel seals is more common than many ppl think. The bigger the car, the more likely they are to leak, and in pick ups, I see many 1/2 tons and gobs of semi-floating 3/4 tons (Ford E-250 vans with rear discs have semi floating rears) that have oil seal leaks. The more weight on the vehicle, the more likely that the seal will give out. Usually the right rear too...since that is where most of your power goes.
The good news is that it is a fairly simple fix, and a good time to upgrade your diff oil to synthetic. I would recomend replacing the other one at the same time, and if you can get national seals, (try NAPA) then you'll be better off.