Brake pressure

brotherGood

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Well..its looking like in addition to the Olds I'm going to end up with my old AHB back. Since this is the case, I'm going to finally try to figure out the brake issue.

Currently, it has next to no pressure when bleeding the rears, either side. Also, it feels like the brakes drag more and more the longer I drive it. At this point, I cannot remember when the problem started.

It has all new lines all the way around (3/16"), new hoses, calipers were replaced about 8 years ago, new drums, k car booster and master cylinder, new proportioning valve/dist. block (I know its not related to this as it was the reason for the new block)

In working on the Olds, I've thought maybe the line running to the rear needs to be bigger..similarly to how it is on the Olds. Hydraulics (I would think) would be the same. I also wondered if maybe the issue was the k car booster. More specifically the pushrod length not being correct.

Does anyone have any ideas?

Thanks.
 

Aspen500

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The 3/16" line is fine. Some GM's use 1/4" but I'm not sure why. Just going off your description, my first guess would be the rear hose is restricted. Did you have the problem before the hose was replaced? It may be fine but new just means new, it doesn't necessarily mean good (one of my tech school instructors told me that almost 40 years ago, and he was right). You could try applying the brakes a couple times and see if the rear drags with the rear tires off the ground. If it does, crack a bleeder and see if the free up. If not, do it again and crack an axle line at the hose. Another possible is, as you mentioned, if the master pushrod is a bit too long, it'll block or partially block the compensation orifice and cause lack of fluid when bleeding and also the brakes to drag.. They get worse when it heats up and fluid expands. If it can't go back to the master reservoir, the brakes apply more and more as they get hotter. That usually affects the front disc more than rear drums (the dragging) though.

You could also try cracking the line before the hose and see if you get fluid flow. If not, then go forward to the next connection, etc.
 

brotherGood

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The dragging kinda feels like the fronts, but to be honest it's been so long since I've driven it. It felt like slower version of when my booster was going out, and the fronts just slowly started locking up (almost as if there wasn't enough there to retract the piston/pad off the rotors)

We'd thought about going back to the OE style booster/master (pretty sure I have an old one lying around) but when you mention the hose, I honestly feel like the last time I was up in there I noticed the hose had come out of the bracket holding it. I mean, if that's the case then surely it may have a pinch..not sure why I didn't do anything about it at the time.

I've gotta get up in there at some point soon to put different pinion shims in, so I may just cut the old hose off and put a new one in. The one thats in there was new about 6 years ago, but the fact that it's hanging essentially makes me wonder.

Maybe I just needed time away from this thing..ha
 

Aspen500

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What can happen to the hoses is, the inner liner delaminates and makes basically a flap or one way valve. Usually it only affects brake pressure release but it can cause a fluid restriction when trying to bleed the brakes. As a test if you feel a drag on the fronts, loosen the nuts holding the master to the booster a bit and see then if the brakes release fully. If so, the pushrod adjustment is too long. I have run into that on totally stock vehicles, usually with high miles. The brakes drag and after driving a few miles they almost lock up from the master not be able to retract totally. I think what happens is the diaphragm plate in the booster gets "bent" outwards after a lot of use and makes the pushrod too long. On those, a new booster takes care of the problem.

With a digital caliper you can check the pushrod depth. It's a few measurements and some subtraction to get the result and I'm too tired right now to type it in :eek:
 
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