Hey all, working on my '88 GXL.

I recently purchased a new brake master cylinder from Napa (took forever and a day to get) and I got something I wasn't expecting... in a good way. I haven't installed it yet.

This thing looks stout. I've never seen an aftermarket "stock" master cylinder that looked this nice.
- The casting is physically bigger on the outside, and the walls appear at first glance to be bigger.
- The bore is the same size as factory for my car. I took it apart and all the holes appear to be properly deburred, and the honing finish is top notch, unlike some of the reman's I've seen.
- The cap is different from literally every other Mazda style master cylinder I've ever seen on an FC (the cap seal is integrated with an external vent like most GM MC caps.)
- It came with the level sensor float which I've never seen new before.

I've replaced about 3 or so FC brake master cylinders before on different occasions (different cars) most of what I can scrounge up for a replacement is either a reman or a reproduction but this thing is neither...

Here are my questions: is this some sort of a unicorn master cylinder? Or has anybody else seen this same style of FC master cylinder before? Is this just an S5 master cylinder? Is this a miata cap? (I don't have one around to compare this to.)

 

Not sure on the cap, but it sure does look like a nice product! Hard to tell from the pictures, but is the master cylinder raw cast or is it painted?

 

I'm 99% sure it's raw cast with a light film of oil on it.

 

The Mazda parts microfiche lists three different p/n's for the S4 NA MC and lists only one p/n for the S5 NA MC. It appears that the S4 NA MC's may vary depending on whether or not you have: 1. w/ ABS, 2. w/o ABS, w/o AAS, and 3. w/o ABS, w/ AAS. Maybe it would be a good idea to recheck your p/n?

I replaced the MC on my S5 NA a couple months ago. The old MC was only about 5 years old. I would sometimes hear a clicking sound when I depressed the brake pedal. I bought a new Beck/Arnley MC (p/n 0728430 ) from Rockauto for $37. The new MC was an improvement in braking performance. It was sort of a PITA to change because there were 4 brake lines connected to it. I had to reuse the old single wire level sensor because the new level sensor had a two wires.

 

I purchased the correct master cylinder for my car (AAS/no ABS.) Again, this isn't the wrong part for my car- it's just cosmetically different (aside from the cap and reservoir which are completely different.)

Here are the Napa part numbers for New master cylinders with reservoir for '88 non turbo coupes:

W/ ABS: NMC M2768
W/O ABS W/ AAS: NMC M92767
W/O ABS, AAS: NMC M2766

On the master cylinder for base model cars, the picture on Napa's website shows an integrated cast reservoir and metal lid like a '60s-'70s Chevy
NMC M2766

 

I'm just saying that when you buy parts from many of these aftermarket parts suppliers it's always good practice to double check to make sure you got the right part. I don't remember how many parts I bought from Rockauto that were misboxed (i.e., correct p/n on box, but wrong part inside box), but it's been more than a few.

You're absolutely right, that MC does look like an AC Delco MC for a '60s-'70s Chevy. Rockauto has an identical new MC listed for the S4 NA (https://www.rockauto.com/en/moreinfo.php?pk=8089580&cc=1192362&jsn=376). These MC's are ACDELCO 18M391446 and RAYBESTOS MC391446 ( w/o Anti-lock Brakes; Except Electronic Adjustable Suspension ) for about $50. I guess the good is no plastic reservoir that can leak, the bad is that you have to remove the cap to check the fluid level. Also, I don't any fluid level sensor on this type of reservoir so the brake warning light might be on all the time.

 

Do all S4's have a brake fluid level sensor?