Prop valve question?

[Paul73]

So I converted the brakes on my 69 roadrunner to front discs (rears are still drums). I put a disc brake master cylinder on and when I drove it around, the brakes sucked so I called the place I ordered the kit from (speedway motors) and they said I needed either a disc brake master cylinder or a disc brake prop valve. I already had a disc brake master cylinder so I threw it on there so I wouldn’t have to buy a prop valve but I was told buy a buddy that I need to change the prop valve anyway for disc brakes… my question is if I change the prop valve, do I need to bench bleed the master cylinder again?

 

[steve340]

Have you got a power booster?

 

[Paul73]

I wouldn't think so, but one question. How are the brakes with the new master cylinder?

They weren’t good, but apparently I was supposed to change the prop valve as well which is reason why they didn’t stop well. I’m just using a stock style master cylinder.

 

[Paul73]

Have you got a power booster?

Nope, they’re manual.

 

[Paul73]

Don’t know if anyone is familiar with it but that is the kit I am running.

 

[steve340]

The reason they are not much good is a disc brake needs around 900 psi of line pressure to operate efficiently. It is hard to achieve this with a manual setup.
A drum brake only needs 300 psi so you fit a proportioning valve to the rear drums to limit the pressure.
The front discs need all the line pressure they can get.

 

[Paul73]

The reason they are not much good is a disc brake needs around 900 psi of line pressure to operate efficiently. It is hard to achieve this with a manual setup.
A drum brake only needs 300 psi so you fit a proportioning valve to the rear drums to limit the pressure.
The front discs need all the line pressure they can get.

oh okay… that makes more sense.

 

[steve340]

You really need to fit a vacuum booster or hydro boost - and a proportioning valve to stop the rears locking up.
Look at a modern car and at how many have manual disc brakes - ah that would be none.

 

[R/T Boy 67]

A lot of people use a 15/16" master with manual disc's.

 

[Kern Dog]

Uhhh....

The reason they are not much good is a disc brake needs around 900 psi of line pressure to operate efficiently. It is hard to achieve this with a manual setup.
A drum brake only needs 300 psi so you fit a proportioning valve to the rear drums to limit the pressure.
The front discs need all the line pressure they can get.

While help is appreciated by the members, it needs to be accurate.
The only reason a proportioning valve is needed is to reduce rear line pressure to avoid them locking up before the front.
If neither end of the car locks up the wheels, adding a proportioning valve will not help.
I don't agree with Steve's numbers. My sources have shown you need 600 psi as a minimum for drum brakes.
Regardless....a manual system can produce that if the right sized master cylinder is used.
Hundreds of thousands of A body cars were built with manual front disc/rear drum setups. These are often times the most reliable and effective systems that I have driven. I have manual disc/drum on Jigsaw....

The MC is a 15/16" unit and it stops great. The car weighs 3600 lbs.

Steve340 isn't all wrong though.
I have been working on trying to optimize the brakes in the red car and even with 1250 psi to the calipers, it wasn't enough to feel right. It worked fine with front disc/rear drum but the 4 wheel discs needed more pressure. I had a 73-76 A body single diaphragm brake booster in the car until a few weeks ago when I swapped in a correct 1966-70 B body dual diaphragm. Now, the brakes feel right. I haven't rechecked the pressure but I suspect it is over 1400 now.

I have this car in my shop out back:

It has disc/drum and power brakes. The aftermarket booster crapped out so I swapped in a 66-70 booster like I used in my red car.
Now this dude just grips! The new booster made a huge difference. It has no proportioning valve but it also skids the front tires and not the rear. I see no point in installing a prop valve in this car either.

It is easier to get better stopping ability with a booster but not absolutely necessary.
The reason all new cars have power brakes is for comfort and the ability for people of just about all weights and sizes to stop the vehicle.
A 150-180 lb man is stronger than a 110 lb woman. WE can spend a little more effort to stop the car and don't have to have power brakes.

 

[R/T Boy 67]

Kern Dog. Did you use the Doctor Diff 66-70 type power booster in your swaps?

 

[Kern Dog]

Yes, both were the ones sold by Dr Diff.
I’ve installed just two of them but both have worked quite well.
I don’t know how close they appear to originals but they do work.

 

[R/T Boy 67]

Thanks. Good to know. There seems to be problems with some aftermarket boosters.