Argh, I'm about to break out the sledgehammer. Just finished putting on new brake rotors and pads, trying another tactic to fix my ongoing brake problem which I can't seem to overcome. Here's the deal...

Problem: I have a firm brake pedal, but limited braking action, especially at slower speeds. Harder aplication of the pedal does not result in more stopping force. Impossible to lock up a wheel

Things I've tried or replaced:
1. Bled brakes multiple times with power bleeder. There is no air in the system. Pedal will not pump up and is firm throughout it's travel
2. Replaced the master cylinder. After bleeding it and the brakes..no change. Pedal is firm, good bubbles in the fluid, but still can't lock up the brakes.
3. Adjusted the actuation rod from the pedal to the M/C. That changed the point in the pedal travel where the brakes started to work, but still limited stopping action.
4. Calipers. No leaks, and the brakes don't pull at all, so can't be a caliper.
5. Replaced all the rubber brake lines, thinking that they might be "ballooning". No change
6. Just replaced all four rotors and pads, thinking that maybe the original rotors were glazed. Just back from a test drive, no change.

The only thing left is the proportioning valve. Is there a logical reason that it might be causing these symptoms? Any suggestions would be greatly appreciated.

Jimbo

 

Did you bed the new pads in by doing a few hard 50 to 10 mph stops? Sometimes that helps a lot. If you have a manual brake car, is the master cylinder correct for non-power brakes?

 

Big G,
Yeah, I tried the power stops, as well as dragging the brakes to heat 'em up a little. Didn't have any effect. It's a power brake car, so the replacement M/C was identical to the original. The booster is working as there's a marked diffence with the vacum connected vs off.

Jimbo

 

are your breaks manual or power.... my 69 has manual and does not lock up unless the road is slick... you car does stop?

Good Luck!

 

Blockage/ collapse/ rust in the steel lines?
Incorrect master cylinder?

I don't know much about the proportioning valves, but it looks like an easy adjustment to test.

My 69 is manual brake, and it WILL lock up, especially the fronts.

 

Check your master to be sure you have the correct one. Master cylinders for power brakes are different from non power.

 

It's a power brake car and the problem had a fairly gradual onset. In onther words, the brakes worked fine for many years, then I started to loose braking force slowly. I did change the master cylinder to a power unit, but it didn't make any difference and the braking force applied was identical to the old M/C.

If I have something obstructing the main line (especially to the front brakes), perhaps that is limiting fluid flow. I think that will be my next attempt, to blow out those lines. Any thoughts?

Jimbo

 

How much vacuum is the engine pulling at idle. IIRC the booster will need at least 16" vacuum to operate. If its marginal you'll notice a harder brake pedal effort & less braking power.

Jim

 

Sounds like what mine was doing, it was a bad booster. I had to press real hard to get to stop and NFW it would lockup, pedal was firm and no bubbles. When I pulled the vac line off the booster it still held vacuume. Put the new booster on and it immediately went to stopping properly

 

Good bubbles in the master cyl. fluid? Cant have bubbles in there.

This adjustment is not for changing point of application.Its right or wrong-dont get it too tight.


68 Vette? I'm no brake EXPERT but I dont think you have a proportioning valve-its just a block with a switch that monitors frt. and rear pressure and turns a light on if its different.The catalogs call it a proportioning valve but it justs distributes the fluid.



OK whats wrong with the quote feature?OK I've got it figured out!!!!

Jimbo I think Sixfooter makes the most sense at this point.

 

Listen for a "Hissing" sound from under the dash when you depress the brake pedal...(engine running). Hiss=bad booster.

 

I had the same issue with my '67 C-10 pickup. Once I replaced the booster the problem dissapeared and braking was "like new" again.

 

Thanks all for the advice. As luck would have it, I have a new booster that I was going to put on the '69 project. That's the one item I haven't replaced, but assumed it was working since disconnecting it resulted in even harder pedal with less braking action. I don't hear a hissing sound, but then my cam/header combo is too loud to hear hissing. Thanks again folks.

Jimbo

 

Almost certainly the booster.

 

bad booster ! it's time to go with hydrobooster

 

Is there any way to tell one from the other from just looking at it, or would it need to be taken apart?

 

You have to look at the back of the MC. Non power cars have a 1" bore and power cars have an 1 1/8" bore.