Trux

I am converting a drum car to disk. Can I keep the master cylinder and just use a proportional valve?

I want to say no, just because it seems like you would need alot more pressure too fill a calipe then to fill a wheel cylinder. A proportional valve and just decrease the about of fuild.

Am I right or wrong?

thanks for you help

Trickster

Not sure I follow your question, but I have all disc's on my car and it needs the master cylinder and the proportioning (combo) valve.

Apeiron

Use the master cylinder you have now and change the proportioning valve.

89Warbird

No, a drum brake uses a lot less fluid to engage the shoes compared to a disc that needs a lot more fluid to move the pad to engage the disc. You need a new master cylinder.

DJP87Z28

Just the Proportioning Valve needs to be changed on the Drum to rear Disc converversion. That all I replaced when I did mine and they work great.

MrDude_1

you only need to change the prop valve.

the actual master cyl between the two is the same.

fyrechikyn

im also switching my rear drum on my 91 bird to 98 ls1's. do i need an ls1 prop valve or is there a good aftermarket one i can use?

MrDude_1

i had a 4 paragraph reply typed out, and i lost it.

in a nutshell, the stock one will work, but to get max effectiveness yuo need aftermarket.

even if you managed to plumb the LS1 one in, its intended for ABS systems, therefore the rear brakes would lockup too soon.

fyrechikyn

aftermarket it is then who makes a good one? and where should i install it at?

KagA152

iTrader: (5)

you can get an adjustable one from wilwood at summit for $39.99. I was going to run one, but I decided thats just another thing I dont want to mess with, so I shelled out 70 somethin for a 1LE for my LS1 conversion. Ill sell my wilwood for $40 to your door.

TomP

The guys that said to keep the current m/c are correct. Apeiron dug up a tech article from GM... but let me throw out some background info first:

Between 82 and 88, GM had two different master cylinders. J65 was for 4-wheel disc, with a 1 inch (25.4 mm) bore. J50 was for disk/drum, with only a 24mm bore. Doesn't sound like much, but the J65 m/c was meant to give the driver an increased road feel- which is what any performance driver would want.

Check out Apeiron's find here: https://www.thirdgen.org/techbb2/sho...threadid=58295

I went drum-to-disk and replaced the m/c, and went through hell. (Bled it like hell, wound up being a badly rebuilt m/c whose seal tore. After fighting it forever, I took it back, got another m/c, and all was good.) I say if you don't have to replace it, all the better!

But the prop valve, yeah, you gotta change it. Okay, well, you don't Have to change it... guys here have used the disk/drum prop valve and still have been able to stop their cars. But if you're going to go thru all the trouble of a swap, might as well grab the correct prop valve.

"Supposedly" I read that on a 4wheel disk car, there's no proportioning. 50% goes to front, 50% goes to rear- the prop valve on a J65 4-wheel-disk car is just there to (a) split one front line into two front lines, and (b) give a spot for the brake warning sensor to hang out.

MrDude_1

in my reply i lost, i mentioned that.

if you keep your disc/drum valve, its not ideal, however, it will still stop better then it did before.

there is no "delay"
there is no drag.
there is no problem.
and anyone wanting to debate it is welcome to take my car for a spin.
you dont get the full advantage of your new brakes, but you will never get that with a factory prop valve anyway.
even a stock disc/disc car will have a improvement with a properly setup aftermarket prop valve.


you heard wrong.

if it was split 50/50, then the rears would lockup first, everytime.
thats a big NO-NO from the factory... thats actually why 3rdgen rear brakes dont do squat... they are so front brake bias in the name of saftey that they bearly work at all. this is why a aftermarket prop valve is useful on even a stock car...

its also another reason why you cant out brake a ABS car.
ABS equipped cars have more rear pressure... without ABS theres the chance of the rears locking sooner, but because ABS is on there, it stops the rears from locking, so you get 100% rear brake useage, plus the full fronts.

fyrechikyn

ok so i can use my old m/c and booster where do i put the prop valve?

MrDude_1

how:
https://www.thirdgen.org/newdesign/tech/propvalve.shtml

also, do a search on "wilwood prop* valve" you'll find more install tips and such.


the basic layout is like this:

DJP87Z28

My opinion, use the GM Proportioning Valve for a four wheel disc brakes and avoid the aftermarket Wildwood valve. I made that swap from 87drum rears to 89 disc brake using the stock parts and I have everything working great and that was three years ago.

fyrechikyn

thnx for the info guys i feel more confident i can do my swap now ill still do alot more resarch on the subject before i actually do it also