dr bob

There are a couple possible causes of MC failure. Both manifest themselves in about the same symptoms-- pedal falls slowly. First cause is dirt/crud accumulation, which will damage the cups. Second is from moisture, which either directly causes the crud (rust) from steel parts, or there's direct damage to the cups from rust in the bore. A 'rebuild' may be as simple as cleaning all the accumulated crud out, and replacing the cups on the MC piston. If there's corrosion in the bore, it gets tougher since you get to hone the bore carefully to get a smooth straight surface again. Pits are a deal killer generally, but minor scratches may respond to the hone well. Then there's the challenge of getting cups onto pistons without damage, remembering to include the little check-washers if needed, etc. Then getting the cups into the pistons without damaging the lips of the cups on entry. One little scratch or roll-over on the edge of a cup makes it junk.

Brakes are important enough to justify a new MC, IMHO, unless you are experienced and tooled-up well enough to do a real rebuild and there's no honing required.

This also highlights the need to flush and bleed on a very regular basis. Factory recommends every two years, racing groups suggest between six months and a year. We often forget to do this, and end up paying with hydraulic system damage. Avoid the risk completely: flush and bleed annually or more.

Mrmerlin

Yeah what He said............. thats why I was recommending to replace the master, once there is rust in the bore then there will be a pit, if the master is original then you should just replace it , as it will in all probability be rused on the part of the bore that the seals dont traverse,............. untill you push the pedal all the way to the floor, thats when the rusted portion of the bore cuts the seals and causes the master to fail

CFM928

I rebuilt my MC when I had the engine out last summer. I got the kit from the local NAPA. (The brakes were working OK, but I thought I'd refresh the MC while it was easily accessible...) I discovered (The hard way…) that the replacement piston in the kit was not the same as the original, in that it covered the fluid return hole, which caused my brakes to lock up as the MC warmed up… Use the rubber parts for the piston, but inspect the piston VERY carefully before using it…

Nicole

Since bleeding is part of Master Cylinder replacement, I thought I'd put a cross-reference to the bleeding thread...

https://rennlist.com/forums/928-forum/459567-norcal928-saturday-brake-master-cylinder-replacement-and-bleeding.html

However, note that the 84 does not have ABS and the hydraulics go diagonal, rather than front and rear.

Angel84-928

Well, I shipped my Master Cylinder off today. Hopefully will have it by Fri.

BTW, I love my JetBlue discount in shipping Fedex. I shipped it overnight to Philly for $14.

tailpipe

i could sell mine but i would have no idea if they would fit your master and the boxes they came in were numbered for clutch parts so the part numbers on the boxes are usless for MC. they came in rebestos boxes and i was told at the parts counter they were the last two in stock for canada not that i believe them there has to be more than two kits left but seeing as i have 4 928s that i know they fit in i think i will keep them for now sry

Angel84-928

Thanks for the offer but I've got it handled right now.

Cheers

Angel84-928

Got news that they were able to rebuild the Master Cylinder and it's on its way back by UPS ground. It should arrive by Tuesday. I don't think that $108.00 was bad.

Angel84-928

Got the Master Cylinder back and it looks beautiful:

It got late so tomorrow it goes on. If anyone has an extra Master Cylinder Resevoir Cap they would be willing to part with, I would be very appreciative.

Mrmerlin

if you buy a new cap it should come with a new seal as well. To clean out the resivour you can use some brake cleaner first then rinse out with some simple green then a hot water rinse then slosh some isopropil alcohol a few times this will remove the water then air dry with a hair drier set on cool till the alcohole smell is gone.
I would also suggest that you mask off and spray the master with some epoxy paint before you install it so it wont be all rusty looking in a few months. Make sure the rear master to booster seal is in place as well so you dont have a vacuum leak at the booster.
Use ATE brake fluid when you refill and bleed the system

Angel84-928

Thanks for the pointers!!
I've already done the reservoir cleaning as you outlined in a different post.
I will paint the Master before install, didn't really think about that. I will check the seal too.