Rear brake bias problems - Viper front calipers with rear drums

[Mr Man]

Hi all, Merry Christmas.

I'm trying to iron out some brake issues on my '67.
-Rear tires lock up before the fronts
-Brake warning lamp comes on when I hit the brakes

Here's what I have (nothing is original):
Manual brakes
15/16" master cylinder (new from Autozone, intended for a front disk/rear drum mopar)
Wilwood adjustable prop valve with warning switch
Front: 11.75 rotors, 1st generation Viper 4-piston calipers with AR Engineering brackets
Rear: Stock 11" drums from a 70's van, 10lb in-line residual valve

As for the rears locking up: My proportioning valve is adjusted all the way out to cut as much pressure to the rears as possible, but the rears still lock up first. I think I may need to go to smaller wheel cylinders in the rear. Any thoughts?

And for the warning lamp: I have bled the system repeatedly. The pedal is pretty firm and doesn't lose pressure. It doesn't feel like I have any internal leak in the master cylinder.
My plan is to bench-bleed the master again. If that doesn't work, I'm gonna suspect either a bad master cylinder or just a bad mismatch of parts on my behalf.

I'm interested in hearing from the experts!
Thanks,
Scott

 

[BigBlockMopar]

Imo the 11" rear drums cause too much bias.
Best bet is to install smaller cylinders.

My Dart has C-body calipers upfront with 11.75" rotors.
10" C-body rearaxle shortened to Abody size.
At first I also had rear lockup issues but luckily I was able to 'tune' it away with better/softer tires.

 

[vitamindart]

first that switch on the wildwood prop valve is a brake light switch ( not a warning light switch )
verify the lines are plumbed correctly threw that prop valve. you should be able to just about eliminate all rear braking when its turned in.
I have a similar set up on my valiant 11.75 fronts ( stock calipers ) and the wide 10" rear drums without any issues at all.

 

[pishta]

Tru dat, the switch is a pressure switch. Also, larger cylinders will require more displacement/less pressure of fluid to do the same work, smallers will require more pressure.

 

[Inertia]

If the front brakes have air,, it could help prevent them applying properly..

Here's a quick test to see if there's any air left in either system..
------------

Originally Posted by inertia
A quick, easy check for air,, after you're sure the rear brakes are adjusted properly,, is ,.. with m/c cap on,, have friend pump 2 or 3 times and hold down,,, remove the cap,, have the pedal released as you watch the fluid levels,... any "geyser" ( fluid spouting well above the surface) is compressed air, "springing" fluid back,, front or rear,, and that system needs more bleeding..

-with the exception that,- if the rear brakes are outta adjustment,, the return springs can cause a fluid return from the rears..

setting the emergency can help if you can't adjust the rears..

brake fluid is excellent paint remover,, so don't splash any on your paint.. immed rinse with water if you do..

hope it helps... grant

 

[Mr Man]

first that switch on the wildwood prop valve is a brake light switch ( not a warning light switch )

Well that makes me feel pretty stupid... No wonder the light turns on when I hit the brakes!

I'm sure the lines are plumbed correctly through the valve. That's why I was surprised that I could still lock up the rear brakes.

I'll try some smaller wheel cylinders in the rear and re-bleed the whole system.

Vitamindart, what size master cylinder are you using?

Thanks everyone!
Scott

 

[cudajim]

My rear drums always locked up on me no matter what I tried. I installed an adjustable proportioning valve, installed smaller rear slave cylinders, replaced all spring hardware etc. Nothing worked until I arched the shoes then they never locked afterwards. I have a small bench-top belt sander which took off just enough shoe material at the ends to make it all work.