Trouble with disc brake conversion

[wareagle_1997]

I recently converted my 70 LWB over to disc. I rebuilt the front end, replaced my brake lines and installed a power booster and new master cylinder while I was at it. I kept drums on the back. Everything was professionally vacuum bled.

The problem that I have now is that I get SOME braking, but not good braking. For instance, I can't get the brakes to lock up even if I slam them on going 30 mph. Also, braking does improve some if I pump them, but it doesn't totally resolve the issue.

A friend told me that even though the catalogs tell you that the pedal is the same for disc and drum, 67-72, there is actually a difference between them, specifically with where the pushrod hole is in the pedal, as well as the angle of the mounting brackets. He said that he had to adjust his pushrod all the way out to resolve the issue with the old pedal, but changing out his brake pedal to a 71 disc pedal really resolved the problem.

Currently, my brakes are much worse than they were before I started the conversion. Any ideas?

Thanks.

[crazy longhorn]

From what I have seen, the peddle is the same. I run a 73-up disc(manual setup) in my 69, & have not changed the peddle. The brakes are good in the old truck, but I used all the later pcs (master, prop valve, lines ,wheel cyls,& calipers from the 75 donor). If you have better braking after you pump the peddle, it tells me that you still have air in the lines.....as small leak in the system, or a bad master (I have seen some rebuilts that did not "cut the musterd"). best of luck,crazyL

[Longhorn Man]

Try this;
Pump the pedal and hold it for a min... not just a moment... but for 60 seconds.
If the pedal starts to drop, then bleed the brakes again.
If the pedal stays firm, adjust the rear drums up.
This will do little to nothing on the not very good brake performance.
There are 2 holes on teh brake pedal, one is for power brakes, the other for manual... did you move the rod? Off the top of my head, I THINK the top is for manual, and bottom for power.. but I may well have that bass ackwards.
Did you install a disk brake master cylinder and proportioning valve?

[PanelDeland]

Did you change to a brake rod from a Power boosted truck?The legnth is different.This could be causing you not to get enough travel from the piston in the MC.

[wareagle_1997]

I can't be sure, but I don't believe that it's a pedal problem. I think that the pushrod is going all the way into the master cylinder without the brake pedal hitting the floor. But again, I could be wrong.

I only have one hole on my brake pedal to mount the pushrod.

The rear drums are already adjusted about as tight as we feel comfortable taking them.

I've tried two different master cylinders, power boosters and proportion valves, both for disc brakes, from AutoZone(the prop valve was from LMC). Any thoughts there?

We did replace the pushrod.

A friend noticed that the brakes are good immediately after bleeding and adjustment. Around 24 hours later is when they start going bad. But everything is completely dry. No leak anywhere.

Seems like a hydraulic problem to me, but what could be causing it???

[pissonNOS]

try a new master cylinder, i bet 3 out of 10 i install are faulty right out of the box, oops i missed where you said u installed new ones sorry, also check really close for leaks

[kxnomore]

I had the same problem with my 4 X 4. I finally got the 3/4 ton power booster (and changed the brackets from a junk yard truck). Man what a difference. I was actually stopping about 15 feet short of the stop signs. My old lady would lock up the brakes by accident!

Seriously, I don't know the exact difference between the two boosters, but the 3/4 ton one is way stronger...