Brake Help! Please!

[ctlaw74]

Urgent help needed to enjoy my car...
OK, been doing some upgrades to my 66 Mustang Coupe.
14x7 Magnum 500s upgraded to 17x8 chrome Bullitt wheels.
Upgraded to 1-1/8" Sway Bar, shelby Drop, new KYB shocks.
Front drums to Granada discs.
Bigger sway bar rubs the frame and now I need to grind the frame to make room for it like the factory cut out.

Problem is with the brake set-up.Granada discs in front, factory drums in the rear.
JMC dual master cylinders.
3/4 master cylinder with 2lb residual valve for the front discs.
7/8 master cylinder for the rear drums.

Problem is the rears lock up first. I can not seem to get the fronts to engage first no matter how I adjust the darn thing. I do have a Hurst roll control installed on the fronts, but all the fittings and lines have been checked and it flows clear.

The JMC instructions tell you that you must use a 2lb residual valve for discs. Why?
The instructions say use the 3/4 master cylinder for discs, and 7/8 for drums?
Why?

Does anybody have a clue?

This project has begot so many more projects as it is, I just want to drive the car safely.
Thanks in advance.
Sincerely,
Matthew

 

[dsmGST91]

I'm sorry I can't be too helpfull on your question, but do you still have those 14" Magnums?

 

[bdcardinal]

do you have a proportioning valve? thing is that you cant send equal pressure to drums and discs. there has to be a valve that limits the flow of fluid to the rears when you have discs in the front in order to prevent rear lockup.

 

[degins]

Two things make me think that you have the lines from the MC's ass backwards. Residual pressure valves are for drum brakes. Secondly, The 7/8 MC delivers more fluid and disc brake calipers operation require more fluid volume than do wheel cylinders. Thus the 7/8 side is intended to be connected to the front disc brakes. Agree with bdcardinal that you need a proportioming/distribution valve to allow front brake bias.

 

[ctlaw74]

Good idea...

Thanks for the advice guys, the problem is still unresolved at this point as I had to goto sleep sometime last night. As for the bore sizes of the Master cylinders, I am following the directions that came with the JMC kit. I had heard once beofre about the bore size issue being backwards, but I stupidly follwed the directions. As for the reisdual valve, the master cylinders are higher than the calipers so I don't know why I need any, but the kit said I needed a 2lb one for the discs, and none for the rears.

I might try and swap the master cylinders to see if the bore size makes any difference.

As for the proportioning valve, the kit does not require one as their is a bias bar that you adjust to take the place of a proportioning valve. Or so the kit says. It is on the JMC website as well as the CNC brakes website.
Any other ideas?
As for swapping the MCs do you think that could be a bad idea?

 

[twowheels_1999]

rear lock up

Go to www.mpbrakes.com and get the proper proportioning valve for you application. That will solve your problem.