very sensitive brakes after power disc swap

[speed-racer]

I just swapped power disc brakes on my 68 using the following components for a 1972 c10 2wd....

rebuilt master cylinder: Cardone # 10-1511
rebuilt calipers
early classic brake lines
disc/drum prop. valve from 1972 parts truck
brake booster and related components from 1970 parts truck

All other parts were purchased new for a 72 or were pulled from the parts truck.

My problem is that the pedal only has about 3 inches of travel (still sits on the rubber stop when it comes back) and is VERY sensitive. It seems like it is more sensitive when you slowly apply the brakes......if you apply the quickly, they are somewhat sensitive but are driveable. The pedal is extremely easy to press and does not do much of anything for the first inch or so. I have adjusted the pushrod to master cylinder so that there is play, and can feel a slight bit of slack in the pedal. You can drive the truck if you concentrate on applying as little pressure as possible, but it is absolutely terrible. Please Help!!

[speed-racer]

My master cylinder has two clamps, one from the front and one from the rear. I have noticed that others have one clamp that comes from the side. Which is correct?

[68K20 x Drill]

I can't answer the second question, but find side clamp more convenient.

Fore sensitive brakes, drill a new hole lower in the pedal arm to bolt the pushrod to, you have too much mechanical advantage.

[sick69]

Did you by chance swap the front and rear lines? As in have the front proportioning valve running to the rear and the rear proportioning valve ran to the front? Disks and drums need different pressures to operate. I would suggest double checking your connections to make sure they are running to the correct axle.



Just my thoughts though


Frank

[Old Chevy Mech]

Was your parts truck that you took the disc/drum prop. valve off a 1/2 or 3/4 ton? I'm only asking because I've heard here that its different between 1/2 & 3/4 ton.

Have you tried locking them up and seeing which ones are grabbing first?

Dumb question so dont be insulted please, but do you have the vacuum line connected to manifold vacuum correct?

Someone else can chime in here, but is it possible that the booster if old & used could be sticking and suddenly releasing giving the sensitivity? Just a shot in the dark..


Frank

[ChevLoRay]

My truck had power brakes before the conversion to disc on the front. No change in pedal pressure, but the proportioning valve must be doing the job in keeping the rears from locking before the fronts.

I think the suggestion to check the leverage is spot-on. I remember Chrysler products in the early-mid 60's having really touchy brakes. I attributed that to their design but I hated driving a customer's car when it was a Mopar with power brakes.

I understand your concern. Hope the suggestion above helps you.

[speed-racer]

I thought about the mechanical advantage, but I did measure the holes in both power and manual brake pedals and they appear to be identicle. I think this may help fix my problem, but it is so severe that I am not sure it is a true fix. Also, my pedal travel would decrease after redrilling the hole. It already feels to me like I dont have enough travel as it is. It just feels like the pedal does nothing for the first 50% of travel, then your in the windshield. Oh yeah, at the point where the brakes lock (which takes no force at all to get to), there is no pedal movement left. Its like you are on the floor....but most trucks I have driven require a lot of force to get the pedal bottomed out, even with power brakes. Thanks for the help guys....any other input would be greatly appreciated.

[ratrod67]

Just curious. You are using a 70 brake booster. That's originally for drum brakes right? Is there a difference in boosters that would have been in a drum brake truck compared to a disc brake truck?

[speed-racer]

I dont think there is, hopefully someone who does know will chime in soon. Its very possible.

[jpabberton]

I can private message you a great article from the October 2004 article on brake troubleshooting from Classic Trucks (www.classictrucksweb.com). It helped me out. Its too big to post here.

[jamie72]

sopnds like the rod between the booster and mastercyinder might bo too long????

[speed-racer]

Wouldn't my brakes drag if the pushrod was out of adjustment? If not, what is the best method for adjustment?

[Boog]

Have you jacked up the front end to see if the brakes are dragging? I mean jack it up, press the brake pedal and release then try rolling the wheel by hand for drag. Just curious.
Also the rear shoes have to be adjusted proper to get normal brake pedal travel. This might explain no response for the first inch or so of travel. Worth checking.

[speed-racer]

I have not adjusted the rear brakes. I also had trouble bleeding them. Could the rear brakes not being bled properly cause this issue? I will also try to adjust them.

[VetteVet]

Here is a great site on brakes.
http://www.hotrodheaven.com/tech/bra...kes1_index.htm

[Boog]

By all means. Adjust the brakes before starting the bleeding process. The further away the shoes are from the drums the more pedal travel you will have.

[speed-racer]

After reading the brake article that was posted, I realized that I may have left out some important information. The truck has a genIII 5.3 V8, which may have more vacuum than a 1970 v8. Could this be part of my problem?

[sick69]

Originally Posted by speed-racer

I have not adjusted the rear brakes. I also had trouble bleeding them. Could the rear brakes not being bled properly cause this issue? I will also try to adjust them.

Did you bench or pressure bleed the system after the conversion? I know normally air will cause spongy pedal but,,,,,,
Might think about starting from square one and checking every connection,hose,fitting and part for proper installation and operation. It's got to be something in the system you just installed.

I don't think drilling another hole in your pedal is a safe idea. What happens when you figure all this out and your brakes are normal and you have to get on them quick and hard for an emergency stop and the pedal breaks off!!!!!!!

Thats right,,,,,YOUR TOAST! I won't even mention the liability and legality of it if it goes to court after hitting someone.



Frank

[Boog]

That's a lot of good info VV.

[speed-racer]

That was very good info.......and it made me think that the 5.3 swap could be causing some of the issues. Its been in the truck for 18000 miles, but always with manual brakes.

[crazy longhorn]

Is the master cyl listed for drum , or disc brakes?

[bob1967]

I wish you luck, I just went through the same BS on my 67. I purchased a power dic brake conversion kit from performance online ( wouldn't recommend ) and my rears would not bleed. I replaced all the rear brake components and still nothing. I gave up and took it to the shop, they replaced the prop valve and the master and finally got it to work now the rear shoots fluid across the truck. Might want to check the prop and master.

GOOD LUCK

[speed-racer]

The master cylinder was purchased for a 72, can somebody confirm that it is correct?

[Eichorst69]

I remember having this same exact proplem when I replaced my front power drum brakes on my 69. It drove me nut's until I thought to adjust my rear brakes and that solved the problem.
I would suggest adjusting then bleeding the rear's before doing anything else.

I don't believe vacume is the problem and the master cylinder you have is the right one.

Rob

[speed-racer]

Boog, you and Eichorst nailed it. I adjusted and bled the crap out of the rear brakes and everything is perfect now. Thank you guys for all your help.