Are you saying you did a rear drum to disc conversion or that you replaced the front calipers?

If just the front calipers, why?
If so, what brand and part number did you replace them with?

Sounds like your power brake booster isn't contributing as it should or maybe a bleeder valve is not closing all the way. Make sure the vac line for the booster didn't get knocked loose.

Also, if you did a rear drum to disc conversion, I THINK you may need a different front/rear proportioning valve...don't remember

 

Janmar you are all described in great detail, thank you, I follow all the above rules, but it did not help. Previous master cylinder flowed in the place where it joins the brake booster. Can you say in the brake booster has penetrated the brake fluid and therefore engine "suffocating"? Then I have to try to put another, it can solve the problem.

 

Derf, I decided to leave the rear brake cylinders. The system seemingly tight, though, and the brake lines it seems that never changed in 23 years. But while I suspect the brake booster.

 

Ensure the vac line to the booster is connected at both ends and has no leaks.

1)Check for pad rotor contact in the front
80% of stopping effort here

2)Check to ensure DONOR master cylinder good
If still leaking into booster, it is still an issue. It may have other seal issues too.
If pedal remains low, i suspect donor may also bad
Low pedal to me still leans towards master cyl problem (which you apparently had before but didn't know (?))

Power brake booster

Enough brake fluid in a brake booster will eventually eat through the seals, the diaphragm will be wrecked and the vac will no longer be held. If your engine is suffocating, it sounds like it may be pulling a bunch of brake fluid in through the vac line from inside the booster.
____
Pretty much everything I have read says that a modern brake booster gone bad leads to a hard brake pedal -- the equivalent of manual brakes -- because that's what you have with no power booster.

So ultimately I think you should check out the things above.
I think your booster is likely bad, but not the only cause of braking issues.

See link below on power brake boosters.

Diagnose Power Brakes

 

Thx Derf. I'll write when I find the reason

 

I just want to add to the brake bleeding procedure. You should always start with the wheel farthest from the master cylinder first, then move to the next wheel farthest from the master cylinder, and so on until you come to the last wheel.

If you replaced the master cylinder you need to bench bleed it before you put it on the car, to make sure you get all the air out of the master cylinder. If you look how it's oriented when installed on the vehicle if there is any air trapped in the master cylinder it will be forced to the front most tip as it is the highest point on the system. You can still bleed the master cylinder on the vehicle you just need to separate it from the booster and try to make it level, then start the bench bleed process. Here is a really informative video on how to .

Now if you SL1 has ABS you need to make sure you properly bleed the ABS unit too. Depending on the ABS system you have the procedure can be different. Do a Google search for the procedure relevant to your ABS system. It's not hard and can be done without a scan tool.

My best guess is you still have air trapped in the master cylinder, especially if you replaced it.

Best of luck.

 

Excellent post/addition to our library of knowledge.

Wouldn't the trapped air lead to a hard pedal?

Hmmmm .....Come to think of it ---Koster, did the the pedal feel HARD or SOFT when it went almost to the floor?