No power brakes after an E28 Mater Cylinder install?

[540ig5]

'86 325es with ABS

I ran some new brake lines on my car and decided to replace the MC as well as my old unit was acting a bit suspect (sometimes the pedal would sink down at a stop light). I had a used MC from a 528e laying around, so i put it in. From the limited information I was able to find online, it should be a direct bolt on. It did bolt right in and the brake lines lined up no problem. (I did however notice that the inside of the rod that get's pushed in are a bit different, not sure if that's a big deal or not). No issues bleeding the brakes. However, when I test drove the car, I had no power brakes. The pedal is very stiff, I'm only able to push it 1/4 way down (basically it travels and feels the same with the car running or not). Check valve is ok, hoses are ok, no vacuum leaks as far as I can tell. There was no issue with the booster before the swap. I did unbolt the booster and tried to get it out of the car for about half hour, until I gave up and bolted it back in. My next step is to put the old MC in and see what happens. Any ideas what the issue could be?

[M-technik-3]

Larger cylinder gives increase in the amount of effort required to push the pedal. How do you know you are not getting any power assist? In the e30 M3 world many used to upgrade to the E28 master cylinder but these days most have gone back to stock due to the amount of effort required while applying brakes, it can get tiring on the leg.

[540ig5]

I'm basing the no power assist on the fact that the pedal is very stiff, no notable change in pedal feel what the engine is running or not, and also I'm not able to lock the brakes/kick the ABS on when I try to do an emergency type stop. The car does stop, but is it just slowly glides to a stop. Even if I slam on the breaks with full force from about 15 miles an hour, the brake distance is like 10 feet.

[LJ851]

Did you install the O-ring seal between the master and the booster?

[agent]

Please do not ask E30-specific questions in General Discussion. If you don't know where to ask your question, post it in General Technical and we'll move it to the appropriate subforum if necessary. Thanks in advance.

[540ig5]

Did you install the O-ring seal between the master and the booster?

Yes, but not a new one.