STUMPED! Stiff brake pedal on S50 swap. (325ix/735il booster, 735i/M5 MC setup)

[PutterMcGavin]

So I’m stumped. Pulled everything apart, put in a different booster and same problem. System is a E30/E32 325ix/735il brake booster with M5/735il MC. iX spacer used with iX clevis. Basically symptoms are as follows:

- HARD brake pedal with little to no movement when pressed (car on OR off it is the same)
- NO brake assist at all. Brakes VERY tough to engage. ABS does NOT engage (light is OFF)
- Slight dragging of brake pads on rotors
- When pushing hard on brakes can see MC “moving” and pushing against TB intake boot slightly

Tests done:
- Pump brakes up, hold brake, start car, brake pedal does NOT go down or move.
- Brakes beautifully bled. No air at all in the system. While bleeding brake pedal WOULD go down to the floor, return and felt really good like it should.
- Tested check valve. Works great.
- Tested unplugging check valve from intake manifold. Car would stumble when uplugged, idled fine when plugged back in.

All things seem to be pointing to the brake booster. I however JUST replaced the brake booster with a known good one. The one I replaced was losing vacuum through the diaphragm causing an idle issue so I knew it was bad.

I’m thinking it might be MC related for a couple reasons: A) booster seems to be ok B) when I separated the MC and bad booster it was REALLY stuck to it. I had to get creative in prying it off of the brake booster studs so it popped off C) dragging brakes seem to indicate pressure is being held by the MC and not releasing back D) not matter how hard I push, the pedal will barely move (I do 3 sets of 230x15 calve raises so I think I the seat brackets would break pushing into the seat before I would...).

Thoughts on anything else to check? Not trying to throw parts at it but might just get a whole other MC and booster if I can’t figure it out.

[DEV0 E30]

Are you positive the check-valve isn't backwards? I did that once.

If not, you may be right about the MC.

[PutterMcGavin]

Are you positive the check-valve isn't backwards? I did that once.

If not, you may be right about the MC.

Pretty sure. If I unplug it from the intake manifold and blow into the booster I can't but when I try to suck air in it will let me.

[Gooch]

It may be silly, but is the o-ring installed on the master that seals it and the booster? I left mine off by accident on my E28 vacuum brake conversion and had the same symptoms.

[PutterMcGavin]

It may be silly, but is the o-ring installed on the master that seals it and the booster? I left mine off by accident on my E28 vacuum brake conversion and had the same symptoms.

Wouldn't that manifest itself as a vacuum leak and the idle faltering from manifold vacuum pulling extra air?

I don't hear any "hissing" or sucking of air but maybe I'll hit it with some carb cleaner on the outside to see if it does anything.

[Hoffbrohaus]

Any updates on this? I'm having the same issues. Replaced booster, flushed system, changed pads, and I just can't lock them up over 40mph. MC is from a 735 also. I noticed when the car is on and someone pumps the brake I can see air bubbles in the resevoir and hear a squeaking noise.

The PO did not have the o-ring on between the MC & booster. I put a new one on from O'rielly auto parts, so just one that looked like it fit, maybe it's not the right size and still not sealing?

[PutterMcGavin]

I ended up loosening the lines at the master cylinder and bled there and then traced both lines from the MC back to the ABS unit and made sure fluid was coming out there properly as well. It think it was a blockage somewhere in the line and/or too much air since I had the MC out when reinstalling.