76 Eldorado brakes

[cadillacrandy]

I replaced the master cylinder today. The old one was leaking and loosing fluid from the rear chamber. when doing this job i pulled the rod,retainer spring, return spring, and plastic washer.
I bench bled the master cylinder prior to reinstalling it on the car. I drove the car a few miles and all seemed fine until I was about to turn back into the dive and I noticed little need to brake. After getting it parked I noticed a bit of smoked from one of the wheels. It was obvious that a brake was locked up on the rear. I was not having any problems with the brakes before with the exception of the leaking master cylinder. Any thoughts on where I might look first
Thanks Randy.
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[TJ Hopland]

Make sure everything is back correct related to the pushrod in the booster.  If something went wrong putting that stuff back maybe the rod isn't in the centering hole and isn't letting it fully release.   Some of those parts you describe don't sound familiar but I could be remembering the one I did in my Suburban last winter which was a 96 so they could have got rid of a few parts in that 20 year span.   I think the hydroboost was a fairly new animal in 76 so it may be different than the 80 and 96 I have worked on most recently.   

Other thing that comes to mind is are you sure its the correct MC?   Compare the end where it goes into the booster to make sure the depth and hole is in the same spot.  Same as above if something isn't correct there it may not be releasing. 

Last thought is are you sure its for a disc disc application?   In that era that would be an oddball app since most cars were disc /drum.   Drums have a pressure valve in em that keeps a little pressure in the system.  Disc does not have that valve.    Usually a disc drum MC will have a large and small section in the res.   A disc disc should have the same size large chambers for both sections.    Aftermarket / hotrod MC you can usually remove that valve if its not needed.  Not sure if that is easy to do in factory style ones.   One way to tell if its a pressure issue is open the bleeder on the wheel that is dragging.  IF that fixes it you know its holding pressure which would be a internal failure in a hose or a issue with the MC.   Unless its new the hose is a possibility in your case since you were messing with the system.  You could have knocked some stuff loose in the process.

[cadillacrandy]

Thank you TJ, I thought I would check the push rod issue first. The new MC looks to be exactly like the original, nothing aftermarket or hot rod parts type of deal there. I also thought about opening the locked wheels valve again to see if there might be some contaminant or air. It is the one furthest away from the MC. Not that it matters but maybe it does.
TJ, i am not sure what you mean about " One way to tell if its a pressure issue is open the bleeder on the wheel that is dragging.  IF that fixes it you know its holding pressure which would be a internal failure in a hose or a issue with the MC.   Unless its new the hose is a possibility in your case since you were messing with the system.  You could have knocked some stuff loose in the process."

THANK YOU Randy

[walt chomosh #23510]

Randy,
  Lack of(or not enough) free play in the MC rod will definitely lock the brakes up. I've experienced this on a number of different vehicles,both cars and motorcycles.....walt...Tulsa,ok

[cadillacrandy]

Thanks Walt.  I dont like the feel of the peddle when I press it. It kinda has a vibrating feeling to it when pressed and I hear a squalling sound when pressed hard. It is a re-manufactured unit so maybe that is the problem. I have ordered a NEW MC already. Lets see if that is the fix. 
Walt, when you say ROD in the MC are you meaning directly inside the MC or the push rod from the Hydro unit? The Hydo unit seems pretty basic. The rod goes in the cylinder with a plastic washer attached to it. The spring goes over the rod and is kept in place by a retainer that is kind of star shaped and the rod goes in that retainers center hole.

thank you Randy

[TJ Hopland]

If its pressure of some sort in the system causing the brake to drag opening the bleed valve on that wheel will tell you if its a pressure issue or not.  If its pressure opening the valve will release it.  If its not a pressure issue opening the valve won't have any effect and you are just looking for a mechanical issue which there could be many in a rear disc with integral parking brake.

If its pressure then the next question is why.   A common cause is a internally failed hose.   The failure acts like a check valve.  The 100's or 1000's of pressure from when you step on the pedal can get by the bad spot and make the brakes work but when you release it stops the fluid from being able to flow back.   Similar thing can happen if the MC isn't able to release all the way, it thinks you have not fully released the brakes so its keeping some pressure on.   Why only one rear wheel?   Could be lots of reasons especially since you have rear disc with integral parking brakes.   Lots of extra parts in those that could get stuck.   

[cadillacrandy]

I think I had mentioned that when I replaced the MC I did so with a re manufactured one. After checking this and checking that, I thought that the issue just seemed to be at the MC. So, at that point I decided to take that Re manufactured unit off and replace with a new (non re manufactured)one. The brakes seem to be working just fine.

Thank you for all you help.