Brake pedal bleed down

[Madgyver]

I'm not sure if I asked this before.

it's a 70 with drums all around. PO installed a vaccum booster and bracket. There is no proportioning valve and no H-block.
I have a problem with intermittent pedal bleed down. There are no visible leaks so I'm thinking internal with the MC.

The brakes need a go over, that I know but what would cause the pedal to bleed down intermittently?

Some days it never happens and then once in awhile it would bleed down at traffic stops.

 

[El Kabong]

I think you're on the right track with the M/C. Bleed down with no leaks anywhere & no fluid loss indicates it's leaking past the piston back into the reservoir.

Try holding light pressure on the pedal & see if it bleeds down sooner. The seals in the M/C work better under pressure, so light pressure sometimes will show the problem earlier. While you're sitting at a stoplight, you're probably not pushing as hard as you are while stopping, so that's a likely place for it to show up.

 

[Broncobowsher]

Bad master cylinder. You are right, it is internal
The cup seals with pressure in the system. It is damaged (probably along with the bore) and fluid bleeds around the cup.

Generally a light and slow apply will cause a weak master cylinder to bleed down while a sudden hard apply spikes the pressure real fast and forces the cup to conform to the damaged bore.

No matter what, a intermittent sinking pedal with no fluid loss is a bad master cylinder.

 

[El Kabong]

Usually they get worse pretty soon after this begins. You want to get on it & get it repaired before it fails at a critical stop.