One year ago I happened to drive my Wife’s 1991 3.9 Dakota Sport SE and noticed that the brake pedal would intermittently sink to the floor, and on the next application the pedal felt good – I reckoned that the 30 year old master cylinder should be changed. As so often happens when a can of worms is opened - I had to change the rear wheel cylinders, all the lines; and replaced the shoes, pads, and bleeders. I deleted the combination valve and the RWAL hydraulic valve (bleeder completely rusted away and the valve is discontinued) to prepare for the installation of a manual proportioning valve. I bench bled the master, installed it - and then flushed at least 2 quarts of brake fluid through the system at the bleeders, and … the pedal sunk to the floor – I tried gravity – no luck, then I considered that the new master could be at fault, so I flushed the old master, bench bled it, swapped it for the original and now I have a good pedal with the engine off, but when I start the engine – the pedal sinks to the floor. I removed the vacuum line to the booster (and plugged the line) started engine, good pedal, reconnect the line – pedal sinks to the floor. I have considered replacing the rear hose and the front brake hoses but flow at the bleeder is fine - at this point I do not want to throw parts at it (new parts may well be defective as the M/C was) and the pedal is not spongy - it just sinks like a stone. What the heck did I miss?

 

You've missed that the booster is going bad. Read the ... "I removed the vacuum line to the booster (and plugged the line) started engine, good pedal, reconnect the line - pedal sinks to the floor". This screams a bad booster.

Probably the old master was seeping into the booster, and it's rotted out one of the valves (the check valve that keeps it from keeping on applying vacuum), so that it keeps sucking the pedal down.

And no, the new M/C may not have been defective.

I would put it back on with the new booster, since the old master cylinder is probably seeping and will kill the booster.

Check at about 1:45 in this video to see how the vacuum is supposed to work, and by extension, how yours is failing.

RwP

 

Thanks, I considered the Booster, but any time I have witness a booster gone bad it behaved like there was no boost - a hard pedal - not a sink to the floor condition, and when removing the vacuum hose there was some evidence to brake fluid at the hose, or paint damage caused by leaking brake fluid below the master - however, as Will said ‘There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy’, and just because I ain't see it ...
I will think some more about how the this could have occurred, this has been a big help, and super fast.
Pete.

 

*dry* There's two check valves. One will pull it down, one will push it back up.

The one that pulls it down is what's acting up.

Remember, it's a balance; either side, it fails.

Besides, you proved that was it by it NOT pulling down with vacuum applied.

RwP