Difference between ATE master cylinders?

[phenobarb1]

I'm finally getting around to sorting out my brakes after a few months of delay, and nervous driving. The brakes have been decent, but sink a bit when stopped at a red light light, but pressure returns after pumping them. I suspect the seals in the master cylinder are going/gone, no big deal. My problem is figuring out the difference between two different ATE M/Cs that are available from Pelican. The first is for cars ending in production date 4/87, and the part number is 34-31-1-154-918-M4. The second ATE M/C they have is for cars produced from 5/87 forward, and its part number is 34-31-1-157-206-M4.

My car was made in 5/87, but the M/C looks more similar to the earlier version, having only the on protrusion on it's underside (i am judging this by just feeling around under it). Not really sure what the second underside protrusion is for on the later model, or what the differences are between them. There is also a difference of $50 dollars between them. So, just looking for some input so i don't order the wrong part. Any thoughts?

Also noticed that the vacuum hose to the brake booster seems to be a bit collapsed where it makes its turn, and wondering if that might be the issue rather than the M/C (would be nice if that were the case, but I'm not optimistic).

I'm an idiot.

[jlevie]

The pedal sinking when you hold pressure on the pedal is a leak at a line or caliper or a bad master cylinder. Your car should take 34-31-1-157-206-M4.

[phenobarb1]

The pedal sinking when you hold pressure on the pedal is a leak at a line or caliper or a bad master cylinder. Your car should take 34-31-1-157-206-M4.

Yeah, that is what I suspected. I don't appear to be losing any brake fluid, so I think it is probably a bad seal in the master cylinder.

I know the 34-31-1-157-206-M4 M/C should be the right one for my car, according to the production date, but at least in the pictures of it on Pelican's website, 34-31-1-154-918-M4's underside seems more like the one I currently have. Maybe they just have bad or inaccurate pictures, I don't know. That second protrusion (stop bolt?) is what is confusing me, as my current M/C doesn't seem to have that.

[Sagaris]

In my experiences the Ate master cylinders are cast iron while the Girlings are cast aluminum in case you are anal about weight savings.

[phenobarb1]

In my experiences the Ate master cylinders are cast iron while the Girlings are cast aluminum in case you are anal about weight savings.

I do admire the lunatic lengths that people will go to for weight reduction. For now though, I think I'll just shoot for functionality. I remember reading an article years ago about a guy who removed the trunk latch on his Porsche 928 to save weight. When people start measuring weight reduction in grams, it blows my mind.

So, does anyone know if their is a difference in piston size between 34-31-1-157-206-M4 and 34-31-1-154-918-M4 ATE M/C's? Just curious as to why BMW would have switched the part, and whether they are interchangeable? Whatever the story is, I guess I'll just order the 34-31-1-157-206-M4 and get if over with.