Taking apart brake master cylinder, and how I spent my weekend
 

Hi guys -

First, I'm looking for any insight on taking apart the FD stock brake master cylinder. More after the story.

So, been working on my buddy Jon's FD for about a year now - basically building a chassis up from nothing. Got it running last weekend, and did the brakes this weekend.

Put the calipers, freshly turned discs, and new pads on the car. Used my Motive power bleeder and a vacuum bleeder at each caliper (and Speedbleeders, overkill). I was able to bleed the whole system but I noticed -

- the front calipers didn't move at all. Stomp the brakes and you could spin the rotor by hand.
- Rear brakes did fine
- The brake pedal would go down about halfway then feel like it hit a SOLID WALL. You could NOT push past it.

With a rear bleed nipple open, you COULD push past the "wall" but you could still feel it and going past took some effort.

We tried a few things - bench bled the master cylinder, opened lines and checked for pressure at different parts of the system, etc. The master cylinder was on his old car when it was wrecked (long story) about 2 years ago and has sat since then.

Finally we swapped another master cylinder in from the rolling shell that we built up and BINGO - brakes bled like normal, all calipers working, and the pedal feels totally normal. You push down about an inch or something and get firm resistance - not a wall, but firm pushback.

So, I'd like to see what's wrong with the bad master cylinder. Doing some reading I found that people have stated that a valve inside will stick if it sits, and if you open it up and get the valve moving again you're fine. Thing is I can't figure out how to take the damn thing apart!

There's a snap ring at the back where the rod from the booster goes in - removed that, and removed the washer under that. There's another flat surface under that with 3 very shallow round indentations. It looks like that needs to come out and then the rest of the assembly slides out. But it won't go anywhere, even pumping the piston of the master in and out.

I also removed the lock pin on the side of the master.

Any ideas? If it's simply a stuck piston I'd like to open it up, fix it, and have a good master (it's clean and has no external leaks).

Thanks,
Dale

Pulled 2 or 3 of them apart a year or so ago, 1 was reluctant.

There's no additional bits beyond the circlip to undo. It turned out it was just drag from the two O rings around the piece surrounding the push rod iirc - and surprisingly strong! The hard pedal sounds a bit like the pin was inserted in the front piston groove improperly at one stage maybe? I wouldn't take outside condition as too indicative of the bore shape either.

Thanks for the input! The master has never been taken apart, he bought it with low mileage back in '02 or so and I've been the only one working on the car.

I messed with it last night, still can't get the damn thing to come apart. The seal around the shaft that goes in and out came out, but that didn't get me any closer to anything. Beat on it some with a hammer, that didn't do anything :).

Any other suggestions are appreciated!

Dale

Have you tried compressed air on it?

Not yet, but that's a good idea. I have a feeling it will make a BIG mess if it does pop out :).

I'll try that out tonight.

Dale

I'm guessing that somehow something got in there that swelled the (front) seals and locked them to the wall. I've seen that in race masters, clutch slaves and calipers that were not cleaned and lubed with "brake grease" after manufacture or rebuild.

If you have some brake fluid and a hydraulic pump or empty grease gun, you could use that to force it apart. Since the fluid is almost incompressible, you wouldn't have the energy that would be stored in air if you used that. So it would be less dangerous /messy.

How hard of a pedal can we get with our cars? I just re bled my entire system when upgrading fluid and I didn't notice too much of a difference.

If you didn't change pads, and they've been on there for a while, you may have some pad taper wear which will make the pedal lower.

So, tried compressed air - no dice. It was getting 110psi of compressed air, put it on full blast, pumped it, nothing.

I imagine using a hydraulic force could help but the reservoir is part of the circuit, you would have to fill the reservoir or remove the reservoir and block off the ports for it.

Any other ideas? Anyone actually done this?

Dale

I did this a couple of weeks ago, I had some trouble, but not nearly as much as you appear to be having. I used some channel locks with rubber over the jaws, and a small block of wood against the flange for leverage.. All while the master was in a vise of course.

Thanks! I'll try that tonight. Not wanting to get super medieval if I'm just missing something dumb, like a tab that has to be pressed or something.

Dale

Here's what is inside. Notched soft jaws in a vice might be best to avoid damaging the hard anodised finish, seals do ride inside the large piece with the O rings on the outside.

Cool, that picture is a big help!

I think that rear seal (the 3rd item in the picture from the left) is just stuck big time. I'll try a vice and see if I can pull it out.

Dale

THANK YOU! It's apart!

First off, I put some of these in my vice -


Clamped the hollow rod coming out of the back of the master cylinder in there - firmly but not crazy tight. Got my hammer and tapped on the back flange of the master cylinder, it came out SO easy. I'm embarrassed it took me this long!

The front assembly seemed to have been the culprit, it was sticky in there. After removing the side bolt/retaining pin I had to fiddle with it a bit to get it out.

Cleaned out the bore with a shop rag and brake fluid, lubed everything up with brake fluid, put back together.

I don't know if I'm going to re-install this master since the one in the car seems to be fine (the spare we put in) but may keep it on hand.

Thanks for everyone's input!

Dale

Now you will be able to sleep at night :)

Just cross referencing another post that I think will be handy for others coming across this:
https://www.rx7club.com/3rd-generati.../#post10424226

https://www.rx7club.com/attachments/...d_master_1.jpg
https://www.rx7club.com/attachments/...d_master_2.jpg

Nice! Were you able to take apart, fix, and re-assemble without any new seals or parts or did you have to replace some of the internal components?

Tracked down a NOS Mazda brake master cylinder for the hope chest for my car :).

Dale

Fine scratches at the start of the bore from the metal washer seem common , I know one of these masters was in use and was fine - until an engine bay fire melted the reservoir a bit, but it was still operational even then.....your diagram seems to indicate, no seals bearing on that area in use anyhow. On one bad Alfa in the distant past, did have fluid bypassing the rear seal and filling the booster though!

You could throw a kit at it I guess, but the latest rebuild kit, which should be BCYA-43-41ZA from my scribbled notes from more than a year ago, was almost $Au100 iirc, so it was almost as cheap getting a non-oem master from Rockauto.https://cimg5.ibsrv.net/gimg/www.rx7...0aff17d0d7.jpg
https://cimg7.ibsrv.net/gimg/www.rx7...59a2db6b41.jpg

I've seen a few new master cylinders on Ebay recently. Keep an eye out. Some may be NOS, the one I bought recently was.

Dale

FD brake master cylinder rebuild
 

What up guys, I’m rebuilding my master cylinder and I forgot to take pictures of the tear down. Assembled it all and I have 2 left over seals idk where TF they go.
https://cimg5.ibsrv.net/gimg/www.rx7...c10334b02.jpeg
https://cimg9.ibsrv.net/gimg/www.rx7...f57f4c59e.jpeg
Here are the seals