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I had been struggling with this for a few days. I bought a '85 recently and the guy had it sitting in the garage with something up against the brake lever, keeping the master cylinder piston depressed for about 6 years. It had gotten moisture past the first seal and all the way down to the o-ring on the piston. There was a lot of old brake fluid that had gunked up mixed with a little bit of rust.

The first try:
I used a dental pick to clear out some varnished up fluid and rinsed with hot water, vinegar, CLR, carb cleaner, etc. A bunch of stuff got cleared out and rinsed off, but pulling on the piston with a pliers wasn't helping. It wasn't budging.

2nd try:
Hit the outside with a small MAP gas torch. I figured the seals needed replaced at this point anyways, so not worried about burning the rubber. I put heat on the piston, the housing, and anything else that I could find. I held it in a vise with some soft jaws and pulled on the piston with a needlenose vise grip. The top of the piston sheared off when I was trying to wiggle it around from side to side.:doh:

3rd try:
Fire up the air compressor and put pressure into it from the backside. Got a rubber tip for the air nozzle and placed it where the banjo bolt goes. No dice. I was starting to think I'd need to take it to a machine shop and have it drilled out hopefully without damaging the bore. Or just replace the entire assembly for god knows how much money.:doh:

Solution:
I bought a M10-1.25 bolt from McMaster Carr. It had to be full length threads and have a flange to make a good seal where the banjo bolt went. I got grade 8.8 to make it easier to drill. I cut it down to about .5" worth of threads with a cutoff wheel, and ground down the flange which was too large and was interfering with the little tabs that are cast into the assembly. Once it fit down tight in lieu of the banjo bolt I drilled through the center with a 3/16 drill bit. Then tapped it with a 1/4-28 tap set.

Put a little bit of loctite on the threads of a grease zerk and screw it into your hollow bolt, then put the whole thing in where the banjo bolt goes (make sure to use one of the washers off of the brake line for a good hydraulic seal). Pump it up with a grease gun at around 3,000psi and watch happily as the piston comes out like it's supposed to.:smiley_th

The piston won't come out all of the way. After a certain point the grease will edit through both ports into the reservoir. But it was free enough at that point that I could pull it the rest of the way with my fingers. Cleaned out all of the grease, removed the varnished brake fluid, and Bob's your uncle. I hope no one else has one this stuck and needs to use this method. But just in case, maybe that person can save the headache that I dealt with for about 4 days.
 

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Reminds me of online high pressure leak sealing. We pumped compounds into valves, flanges, etc, at 10,000-30,000psi. The compounds were varieties of teflon, grease/asbestos, plastics, polymers, rubber... we even custom blended with metal shavings, sawdust, dirt, shredded gloves .. whatever it takes. Hot. dirty, nasty work, on the surface of the sun, drilling holes into leaky stuff that's 4500psi and not shut down. Slamming those dog turds into the gun. lol

The power of grease under pressure is amazing. You can break a big slab of iron with it. Or a dab pushed into a crankshaft-transmission pilot bearing will force it right out with a couple of taps of a hammer, it's the best way to do it.
 

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Excellent work on the whole assembly and thanks for the pics, that master cylinder is as good as factory new now!
 

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Yeah.... I meant to say good save before my rant about high pressure grease.

Good Save!
 
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