Brake fade question

[Dawso]

Just when you think you have everything sorted prior to the van nationals this long weekend. I have a brake pedal until I ride it going down a hill and it goes to the floor. Same when I'm sitting at the lights, pedal goes to the floor. My mechanic has tried everything, bench tested the master and calipers, new rear cylinders etc but can't sort it. Has anyone offer me some advice as to how to resolve the problem please.

[Innuendo]

Possibly a hairline crack in the brake booster bakelite. This is situated at the rear of the booster where the input shaft is covered by a rubber boot.
People don't know about these and treat booster movement and installation poorly and they crack or chip. Seen many people lift boosters from the input shaft.

[axistr]

Good call Innuendo, however the pedal at no time should sink all the way to the floor if that's what is happening. I would also check to make sure the master cylinder isn't riding on the booster push rod at rest. It sounds like bypassing of fluid past the master cylinder seals. If your mechanic has checked this properly, then do a couple of easy basic checks you can do yourself. I would start with the following.

1) Start the engine with your foot of the brake pedal and get full vacuum up then turn the engine off. Press down the brake pedal with steady even pressure and hold. If the booster is leaking and after a few seconds the pedal will start to push back, this indicates a vacuum leak. If the pedal slowly sinks half way and feels like its touching metal to metal you have a problem on one hydraulic brake circuit. If it sinks all the way to the floor you have a problem with both circuits.

2) Place a pair of multi grips on the flexible brake hoses. Clamp the hose just enough to stop fluid transfer. Clamp the two front and the one rear at the diff then retest. The pedal should be rock hard and won't go down. If it does then the problem is in the master cylinder or proportioning valve.
If the pedal is rock hard and doesn't sink after a minute then the problem is most likely sticking front caliper slides. If you disconnect one of the three clamps at a time, retest and if the problem reoccurs than you will have isolated the problem to that brake or front or rear. It's possible the caliper pistons, uneven disc rotors wear or pads can also course the same problem but sticking caliper slides are more likely.

Hope this helps.

[Dawso]

Thanks for the good advice and ideas, I wish I had run this up the flag pole before it went to my mechanic. He's thorough but not a break specialist. He did regrease the caliper slides (good call) due to the passenger side wearing not square and more than the drivers side. But did not resolve the problem. He also clamped the quarters but may not have had the knowledge to identify the problem area. I'll send him your comments to see what he says prior to moving on.
Keep on vanin.

[axistr]

I assume your mechanic has done all the usual visual checks already, example, brake hoses expanding under pressure (old hoses), external brake fluid leaks, brake adjustment, air in the system.