S4 Brake Master Cylinder Passage Blocked

[Temmck]

Hello Again,

I completed the restoration and rebuild of the brake master cylinder but when bench bleeding I was not getting any fluid out of the front most port. I disassembled the master cylinder and tried to blow air through the reservoir. I blocked off the rear hole of the reservoir so all the air would go through the front hole but no air will pass.

I can push a plastic tube down through the reservoir fill hole and into the hole in the front of the reservoir. The tube has some flexibility so I can feel it go down the passage a bit. I sprayed brake cleaner down the tube but nothing but clean fluid came up. I also pushed a plastic rod through the bore of the cylinder and into the hole at the end. The hole appears clear. But I don't have anything that can I can push through that will go all the way from cylinder to reservoir.

Has anyone experienced this before? I have it sitting with brake cleaner on either side of the passage to hopefully loosen what's in there.

[nmauduit]

Is it a tandem master cylinder ? if so I suppose you've check the operation (there are threads on the forum) of each circuit, there is a valve for the front outlet (which goes to the rear brakes), the "slave" (intermediate) piston can also cause issues as both hold rubber seals.

[Temmck]

I was able to free up the blockage today. I let the MC sit over night with brake cleaner pooled on either side of the passage that goes from the reservoir to the cylinder. In the morning I was able to use the plastic straw to direct the brake cleaner down the passage and break up the remaining gunk. Nothing else seemed to work.
I have now completed the bench bleed and have the MC back in the car along with rebuilt clutch master and slave cylinders. Plan to bleed the brakes tomorrow and have the car move on its own for the first time in 30 years.

[alan.barker]

Wow,
Well done that man bravo.
Get out and drive it with a big smile
Alan