951Tom

Another test you can do to the booster is with the engine running, push the pedal and hold it, then turn off the engine. The pedal should not drop any when you shut off the engine.

Tom
'87 951

cas951

Before you start to replace the booster you must first understand how it works and diagnose if it is the prolem. i'll try to explain how it works. Sorry kinda long.

Picture a basketball with a diaphram placed directly thru the center of the ball dividing the ball into 2 parts. On one side of the ball you have a hose connected to Engine vacuum and the other side of the ball is just a hole to the atmosphere.

What you have now is :
1. On one side of the diaphram you have Engine Vacuum which could be anywhere from 14-19 psi at Idle or decelaration. This is the side connected to the Master cylinder.
2. The other side you have Atmospheric pressure. This is the side connected to the brake pedal pushrod.

As you press on the brake pedal the brake booster is already providing your brake system braking pressure from your Eng vacuum and braking pressure from atmospheric pressure.

Now to trooubleshoot your Brake booster cylinder, Simply push on your brakes and see if it effects your engine idle drasctically. A leaking booster cylinder diaphram will cause Engine vacuum leak and will effect the way yuour engine idle.

Sorry this was such a long reply.

Stan944

Thank you for your comment. I was more asking for practical knowledge, as my case is not black or white. It's somewhere in between.

I'll do the test suggested by 951Tom. That might show if the air leak I hear after shutting the engine off comes from the booster or not.

Stan944

Do I need to disconnect the brake lines from the MC to replace the booster? Is it OK to bend the brake lines a little to do it? Do I need to bleed the clutch slave cylinder after disconnecting the blue hose at the fluid reservoir?