brake pedle going down 1986 K20

[akart]

Ok, Posted this once before but am not clear. Bled the brakes and pedal is good. Start engine and pedal slowly goes down. Stops good and brakes lock up but the pedal slowly go down. Was told that this was the master cylinder but there is no loss of fluid in the MS unit. I am thinking that this may be a hole in the vacuum boost diaphragm. I'll buy what ever parts are necessary, but?? This came about after I installed rebuilt front calipers and pads. Before that pedal was good. Is this a bleeding issue or what?? Thank You for your advice. Some where in Alaska.

[rich weyand]

Master cylinder. Rubber age hardens as it gets old. The cup on the end of the master cylinder piston is no longer supple enough to seal as the piston compresses, and fluid leaks past back into the reservoir. BTDT.

Yes, it was OK before you changed the brakes, but the bleeding process could have tipped it over the edge. M/C rubber is only good for so long.

So replace the master cylinder. With disks in the front and drums in the back, you need one with the two different sized reservoirs. Some come with the smaller in front, and some with the smaller in back. It doesn't make a difference which you use, because the lines have different size fittings on them, and the front and rear connection holes in the MC body are swapped if the front and rear reservoirs are swapped.

[TrailerTrash]

i had the exact same problem with my 77. it was a bad/leaky master cylinder just like Rich said.

one problem that can be present is everything can be so greasy and oily in the engine bay it can be hard to notice a small leak running down the booster

[TheWhiteElephant]

It's an easy replacement. Just be sure to bench bleed the master cylinder.

[akart]

Question: If my MC is leaking and rubber is bad should not the fluid be going down? It is not and pedal is firm w/engine off. Does this indicate a hole in the diaphragm as the pedal goes down slowly with the engine running?

[rich weyand]

Originally Posted by akart

Question: If my MC is leaking and rubber is bad should not the fluid be going down? It is not and pedal is firm w/engine off. Does this indicate a hole in the diaphragm as the pedal goes down slowly with the engine running?

No, it's not the MC that's leaking. The high-pressure side (the part connected to the lines) is leaking back past the rubber cup on the end of the pressure piston and back into the reservoir. The reservoir is not pressurized.

The picture here points to one of the bad seals. Both seals could be bad.
http://www.agcoauto.com/content/news/p2_articleid/266

[akart]

Man O' man that is a really good attachment. Read it and it addressed my problem. Thanks Rich, will install a new MC.