Braking question

[dldubois]

I've finally been putting a few miles on the 280sl now and  finishing up the last of the projects I've got on the car (not that  it will ever end).    I notice when driving particularly on the highway that after driving awhile (30 mins or more) that I lose power braking.

Is it due to air in the system?

I replaced the master cylinder a couple of years back.

[Pawel66]

Just from the perspective of an amateur who went through issues related to loss of booster aid:
1. When my brake booster said "enough" and went bad - I could hear the air hissing when I pressed the brake - it was easy to determine that it was the booster that collapsed.
2. After booster replacement I had:
- oring gasket between booster and main cylinder misfitted - I bought a new one fitted it correctly and it was ok for two years
- the hose-check valve from intake manifold to booster went bad, I bought a new one and replaced

These were the simple things. I am sure there may be more.

[Shvegel]

That does not sound like a bad booster to me.  brake fade after driving a half an hour leads me to believe you have a caliper or pad that is sticking.  After a half hour of driving the caliper is getting hot enough to boil the brake fluid which makes the pedal spongy until the brakes cool.  If I were diagnosing it I would drive it until the brakes fade and check the temperatures on the brakes on all 4 corners with a infrared thermometer.  If and when you do find the problem make sure to flush and change the brake fluid.

[ja17]

I agree with Pat. Also check for bad brake hoses also, causing the brakes to boil.