Gimme a brake! Front/rear proportioning problem...

[markrinker]

Just did brakes on BOTH trucks in the same week. Ouch! What ever happened to the 'good ole days' when a set of pads would do?

Truck #1 - Calipers, rotors, pads replaced in front.

Truck #2 - Caliper slides, rotors, pads in front, shoes, wheel cylinders in rear.

Truck #1 does not seem to be braking properly - feels like the fronts are doing all the work. That combined with the clue that the rears were still about 80% says they aren't working.

How do you test? Is there a proportioning valve outside the master cylinder that can be replaced?

[Phil Holmen]

Does your e-brake work very good, it could just be that the rear brakes are not set up enough to grab. If the e-brake does not hold at all just try your brakes with the e-brake on, if they work it is just a matter of setting them up.

[markrinker]

E-brake works like a champ. When towing, its very noticeable that the fronts are doing all the work. Backs might be engaging, but not enough.

Update: Shop that did brake work said they adjusted the rears properly and there is 'nothing else to do'. I am thinking that the proportioning valve is faulty, or there may be air in the rear brake line, requiring a bleed.

How does one test brake proportioning?

[MTTwister]

Mark - There should be a Rubber Flex line from the frame to the Diff. You might want to check that for internal collapsing.

[GMCfourX4]

I did a rear-disc conversion on my truck, and feel that the rear discs also aren't doing their fair share (although, with a proportioning valve setup for disc/drum, that would be the obvious problem). I hadn't settled on a solution yet, when I came across some info in the ImpalaSS forums (I also have a '96 Impala). In the Caprice/Impala, GM never manufactured a disc/disc proportioning valve, even after switching the Impala and 9C1 Caprice (Police package) over to disc/disc. The solution found by members of that forum was to remove the valve that greatly reduces pressure/fluid to the rears, and grind the shoulder off of a bolt that restricts fluid flow to the fronts. This simultanteously increases proportioning to the rears and allows fluid to flow faster to the fronts, resulting in greatly increased braking performance (the Impalas stop VERY well from the factory, too). I went to the junkyard to get a proportioning valve to do this to for my Impala (I don't want to ruin the one that's on there if I don't do it right...) I noticed that the proportioning valve is VERY similar to the one on my truck. I am going to be doing this modification to my truck soon, and will let everyone know how it goes. A note of caution - before I swapped to rear discs, I had leaky axle seals that made my drums lockup with very slight pedal pressure, and caused more braking problems, b/c the ABS would kick in early, and it took a LOT of pedal pressure to get the fronts to bite harder once the ABS was working on the rears.... Unless you plan on installing a manual valve farther back on the rear line (or maybe between the regular prop. valve and the ABS controller) this might make the rears bite TOO well on a disc/drum truck.... Either way, I'll let you know how it turns out when I try it on mine. Here's a link to the article on the Impala mod, for those who are interested:
Brake Mod Tech page (http://www.goissca.com/~gofasst/brake_proportioning_mod.htm)

-Chris

[DieselDavy]

Mark,
I suspect your brake repair guys may not be experienced enough. (The proportioning valve is sort of a mystery to us mear mortals!) I had a similar problem with my oldest diesel but really didn't realize it. It took a lot of pressure to brake. When a brake line on the rear started to leak, I took it in to my favorite repair guys to have them fix it. When I went to pick it up, the tech said he "popped" another line while getting my proportioning valve to work. (He said it was stuck!) He was very apologetic for having to replace another line. (I was not complaining. If another line was weak, better to change it now!) I don't really know how he did it but when I got it back, the brakes were the best they have been for many years! The rears seem to break alot more than before. Bottom line: Get a good brake shop to look at it!
Dave

[Turbine Doc]

I enhanced mine by shifting to stainless brake lines, and also knocking out the steel stamped blank plugs in the back of the plate so I could access the "automatic" adjusters.

I set them for a slight drag on the shoes while rotating. I have never had much success with any of the auto adjust featured brakes I've run across over the years, and have always set drag manually after a new shoe install.

[MJEasly]

Chris,

Did you ever upgrade the master cylinder? I think you were looking at the master cylinder from a C3500HD/4-wheel disc diesel truck with the 40mm bore.

BTW, where did you get your Caddy calipers? I found a set loaded from Advance, but they don't include the parking brake lever. Given the unreasonable core charge and lack of parking hardware, I just might order my set from TSM.

Mark

[Kennedy]

I upgraded my Master cyl with great results in pedal feel. The proportioning valve was to be my next project, but never got around to it.

As Tim said, adjust them up a bit tighter. Like trailer brakes, I adjuct dead tight, then back off till they roll free. This gets them about as even as can be.