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Front brakes stay engaged

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Jul/29/2020 at 10:00pm
For the MC DO NOT send it off. It's not rocket science to rebuild one, but it's very difficult to replace an original. Find a local shop, even if it's a couple of hours drive, and hand it to them to rebuild and pick it back up when they are done.
You give it to the major auto parts places and they send it with dozens of others to be rebuilt. You get back what ever comes off the pile that matches the closest and will do - as far as they are concerned. If they have to put plugs in one side and machine the opposite side for line fitting, they just do it. You could be getting a Ford, GM ???? back.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mbwicz Quote  Post ReplyReply Direct Link To This Post Posted: Jul/30/2020 at 4:56am
A quick check to see if the pushrod is too long is to add a washer between the master cylinder mounting flange and the booster it mounts to. This will move the MC away from the booster by the thickness of the washer.
This is not a long term solution, just a way to see if the pushrod is preventing the return of the fluid. If the problem goes away, then get a tool to adjust the pushrod length and you should be good.
Be aware that you will likely notice the difference when you drive with the washer in place, the pedal will be lower. Again, just a temporary method and not a long term solution.
A messy way to confirm that the fluid is not being able to return to the MC is to loosen the front brake fitting at the MC when the brakes are dragging. If there is pressure in the line, then the issue is the MC (check valve or piston not at the 'home' position). If there is not pressure in the line, then either the hose is holding the pressure in the caliper or the caliper is mechanically binding. Just remember that brake fluid strips paint, so be careful to clean any that sprays or drips.

Good luck,
Mike
1970 AMX. Holy Crap its easy to spend money on this car!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote akimmet Quote  Post ReplyReply Direct Link To This Post Posted: Jul/30/2020 at 10:09am
Originally posted by 72_AMX 72_AMX wrote:

Thanks for the follow-up. I did check the Master Cylinder and the small reservoir is in the front. I talked to the mechanic who installed the MC and asked him about the push rod. He stated that there was not a problem that he could tell when he installed it. I still have the original MC and will send it off to be rebuilt just in case. We are going to replace the brake hoses and see if that works. Any ideas on if the hoses are bad could the interior pieces of the hose contaminate the channels in the calipers? Any suggestions on a good brake MC and Caliper rebuilder.


I also do not recommend sending in an original core to a parts store re-builder, you likely won't get your correct core back. Find someone local to rebuild the master cylinder if at all possible.
Bad hoses can indeed cause all of the problems you described. Usually this only happens to really old hoses. However; due to experience, I no longer implicitly trust new parts to not be bad or dirty out of the box.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72_AMX Quote  Post ReplyReply Direct Link To This Post Posted: Jul/30/2020 at 3:32pm
Thanks. I will try the washer this weekend and post a follow-up. Also thanks for the advice on the MC rebuild. I was worried about getting my booster back.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Jul/30/2020 at 10:36pm
Originally posted by mbwicz mbwicz wrote:

A quick check to see if the pushrod is too long is to add a washer between the master cylinder mounting flange and the booster it mounts to.  
Mike

Oooh, nice trick! Of course! Space it out too much, no big deal (for testing). In fact you could probably stack up shims to work out exactly how much too long the current one is, shim it to "perfect", measure the stack of shims and find/make a rod that much shorter.


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Post Options Post Options   Thanks (1) Thanks(1)   Quote Fluffy73 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/31/2020 at 1:57am
Many master cylinders have adjustable push rods.

I guess the easiest test would be simply separate the master cylinder from the booster.  If the brakes release, there's your problem.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rockAMX Quote  Post ReplyReply Direct Link To This Post Posted: Jul/31/2020 at 5:52pm
Also make sure you use shims to center each of the calipers on its rotor. If the caliper is off-center, the disk pad will make contact with the rotor.
DWR
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