Help 4 Wheel drum to 4 Wheel Disc conversion question.

[Pbum5]

I have a '67 Mustang and I'm in the midst of a 4-wheel drum to 4-wheel disc conversion. I have a Wilwood adjustable proportioning valve to reduce the pressure on the rears. I'm getting ready to plumb the brake lines and I have a question.

I'd like to run the lines through the stock pressure distribution block, since the existing lines are in good condition.

I would run the brake line for the rears from the MC to the PROP Valve to the DIST. BLOCK.

The line for the fronts would just go from the new MC to the distribution block. That would be the easiest way to plumb it.

Can I use the stock distribution block in this way? Are there residual pressure valves in the stock distribution block that will prevent the disc brakes from working properly? I think I've heard of this before, that drums need a special valve to keep pressure on them so they don't retract too far. Should I just buy a "T" at an auto parts store and use that to split the fronts, and then run from the MC to the prop valve to the rears? If there are residual pressure valves in the stock dist. block, can they be easily removed? What do they look like?

thanks

 

[CornerCarvin67]

Great question. I'd like to know the answers too! I put in the Baers last year and plumbed it just like you are intending to do... with the factory valve... and the new prop valve to the rears. It just doesn't seem right. I talked to a guy and he said that factory distribution block could be bottlenecking my system. He said to do exactly what you mentioned in the second half of your question.

It's just alot more work I know.... but one of these days I'll get around to doing it that way. It just makes more sense.

 

[reenmachine]

Both of your hunches are correct. That factory "distribution block" is a proportioning valve. It's not adjustable, just factory calibrated for the specific brake setup your car had from the factory. Yes, a bit of residual pressure is required in the lines going to drum brakes in order to keep the shoes from retracting too far from the drums. For example, if you used a disc/disc master cylinder on a drum/drum car, the first pedal push would always be soft as you moved the shoes back out. A factory drum master cylinder may have a check valve built in, and you can buy external residual pressure valves to fine-tune your system. A disc/disc setup should have the appropriate master cylinder, with the line for the front going directly through a 'T' to each side and the line for the rear going through a proportioning valve (adjustable or otherwise) and then on back. The proportioning valve can be anywhere between the master cylinder and the 'T' at the rear axle where the line splits off to each rear wheel. You'll notice the difference when you set it up this way.

 

[Pbum5]

Thanks Reen.
That’s what I needed to know. Your Car is looking very nice by the way. Keep those pics coming

I am using an 84 SVO MC for 4-wheel Disc.

OK- So this is what I am going to do.

- Get T-Line for the front brakes

- Chuck the stock 4-drum distribution block (not really but it’s coming out)
– What do I do with the brake warning wires when I loose the stock distribution block?

- Wilwood Adjustable Prop valve from MC to rear line-
-Does the APV need to be mounted or can it supported/suspended by the steel brake lines?
– What is a good starting point to set it at with 4-wheel discs?

 

[84convertablegt]

– What do I do with the brake warning wires when I loose the stock distribution block?

Buy a brass tee kind of fitting to plumb in that brake warning light.

 

[chepsk8]

I've used MASTERPOWER stuff for a while, they have great tech info on thier site.

I fully agree with the others, ditch the old Prop valve, no good for that application.

In my Cobra with 4-wheel disc (87 MKVII in front, 94 Mustang GT in back), I used the M/C from a 1994 Mustang SVT Cobra, as it's bore matches the needed capacity for the system. I also have a prop valve on the rear brakes. I used the 94 rears, as they also have a built-in parking brake in the calipers.

 

[ron67fb]

Residual pressure valves are in the master cylinder, behind the orifices where the lines are screwed into. You can feel them by sticking a paper clip in the hole - the valve will have a small spring behind it. The brass fitting can be pulled out and the valves removed if you want to use the old drum master, (which you aren't). I've also found out that many drum masters you buy from the store don't even have the valves installed. I'm guessing as they rebuild them they just didn't bother installing new ones.

I'm using the stock disc/drum master, then I have the proportioning valve between the MC and the block. The prop valve is just sitting there by itself, although it wouldn't hurt to hard-mount it as it has its own mounting holes anyway. I used stainless steel braided flex hose to plumb it, piece of cake. As luck(?) would have it, it rained after my rear disc install. Even a slow stop would lock up the rears. I just kept reducing pressure until the lockup went away. Try wetting your street and testing it there. I'm talking slow, like a hard stop from 15 to 20 mph. You can then start testing at higher speeds and harder braking in a safe place like an empty mall parking lot to fine tune it even more, if you want. I didn't feel the need to do so after the impromptu rain testing.

The stock distribution block is not a proportioning valve. Like the name says, it's a distribution block. All it does is split off the circuits between front and rear. There is a small piston inside that stays centered. If one end of the car loses pressure, the piston will slide out of place and activate the warning light. Most people can notice a low pedal well before that light will activate.

Buy a brass tee kind of fitting to plumb in that brake warning light.

That's not going to work because the warning light is a switch that is activated by a plunger inside the stock distribution valve. Just remove the light from under your dash and put it away. You'll know you lost pressure when your pedal gets low anyway; you don't need a light.

FWIW, I did the rear disc swap over a decade ago when SSBC was just started up. The front power disc setup, off of a '67 Cougar, were done even earlier than that. The car is my backup daily driver and my brakes work just fine, and I've had my share of emergency stops.

 

[Hack]

Residual pressure valves are in the master cylinder, behind the orifices where the lines are screwed into. You can feel them by sticking a paper clip in the hole - the valve will have a small spring behind it. The brass fitting can be pulled out and the valves removed if you want to use the old drum master, (which you aren't). I've also found out that many drum masters you buy from the store don't even have the valves installed. I'm guessing as they rebuild them they just didn't bother installing new ones.

If this is true then with a new disc/disc master cylinder, Pbum5 just needs to install the adjustable proportioning valve and he can use the existing distribution block.

Conflicting answers here. I like Ron's answer, because that makes it a lot easier to finish the plumbing. Can anyone else jump in to confirm?

 

[Pbum5]

Thanks ALL.
Anyone have anything else to add?

 

[reenmachine]

Residual pressure valves and proportioning valves are different things.

Residual pressure valves maintain a certain line pressure once the pedal is released. A 2 psi RP valve is sometimes used with disc brakes to cure a soft pedal, and a 10 psi RP valve can be used to overcome drum brake shoe return springs if there is no check valve in the master cylinder.

A proportioning valve controls the "proportion" of line pressure between the front and rear circuits. An adjustable one allows you to tune this pressure ratio to an optimal setting for your particular hardware, traction situation, etc. Stock ones are typically calibrated for the OEM brake hardware delivered on your car from the factory.

I still maintain that, in most cases, it is not appropriate to use your factory distribution block with altered brake hardware. The cleanest thing for you to do is to run a T off the front circuit to your front brakes, and an adjustable proportioning valve on the rear circuit.

 

[ron67fb]

Reen is right, removing the block is the cleanest way. But I'm not clean, I'm lazy, and it works. Two braided hoses, one adapter, and I was done. No cutting, no flaring, no bending and realizing I forgot to install the fitting first, no "piece of f^$$ @#%%#$ ##^#(* it's leaking again!!"

Basically what we all need to learn is that the stock distribution block is not a proportioning valve by any means, at least not on a '67. Take one apart, all you'll find is a piston that activates the brake warning light and nothing else. Factory proportioning comes from matching of components. Front caliper piston area vs. rear wheel cylinder, front vs. rear drum diameter, width, and wheel cylinder diameters, for example, are all designed to work together from the factory. Change any of that and things happen. Different adjustments between front/rear drums can also throw the balance out of whack.

What I'd like to see is an aftermarket progressive prop valve. The usual adjustable ones are more like switches. They allow a certain amount of pressure, then chop, no more pressure for you. Further modulation is not possible to that circuit until you release the pressure. I've seen factory prop valves under some cars mounted on the chassis above the rear axle. A lever extends from the valve, and connects to the axle. Under braking, the chassis rises, the lever extends, and slowly reduces pressure to the rear. Ingenious! A twin master cylinder setup with a balance bar would also be superior but that gets a bit complicated and expensive.

 

[truss]

4 wheel disc brakes...please help

somebody please help me with this....please call me at 208 949 7450....I have granada's on the front and crown victoria on the back and i can't get rear vbrakes....need help...please call....thx,todd