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.

[Dawso]

I picked the van up from the brake specialists last Friday. They tested the master cylinder on the car (it had been previously bench tested elsewhere) and diagnosed a loss in pressure which was due to a burr in the cylinder. They reconditioned it and I now have reasonable brakes but there was still a problem with the pedal depressing. They put a vacuum gauge on the motor and found it was fluctuating at 15 Psi not steady at 20 Psi (I think its Psi). He suggested that one of my cylinders is lacking compression, possibly a valve riding and it should be checked out by the guys that rebuilt the motor (considering it has only done 12,000 km). The short is I get brake vacuum assist at the top of the pedal but because the vacuum in the motor is fluctuating the vacuum cant keep up and the pedal goes down. There you go.

[axistr]

Fluctuation readings from a vacuum gauge at idle generally indicates a leaking intake valve or leaking inlet manifold gasket. It can sometimes be slightly different depending on where the vacuum gauge in connected. However a cam lobe or lifter problem is less likely but can't be ruled out as is a riding rocker. Large cam lobe overlap and unsteady idle will also give fluctuations, and correct checking of the readings on a vacuum gauge can indicate the difference in the problems I have mentioned above. But that can take a reasonable amount of experience with reading vacuum gauges which seams to be a bit of a lost art these days. Tuning engines with a vacuum gauge before EFI was introduced was the norm. I can't see your car from here, but just reading between the lines I wouldn't rule out the brake booster. It sounds like you may have two problems, the master cylinder which has now been rectified and a vacuum problem (either engine or booster)

Quick test at home,

Run the engine and get full vacuum, turn the engine off, place your foot on the brake pedal with moderate steady pressure and hold.
The brake pedal feel shouldn't feel any change over a minute. If the pedal starts to feel like its trying to force your foot up (push back) then you have a problem with the brake booster or booster inlet check valve.

If after one minute the feel doesn't change take your foot off the pedal and slowly pump the pedal up and down a further three or four times. Each time you pump the pedal it should feel a little bit harder to push down. (using vacuum) After that the pedal will feel stiff and harder to depress. This will indicate your booster is working ok. If you can't get at least three pumps before all the booster vacuum is lost then you have a booster problem. Note, you need to have full vacuum in the booster at the time of turning the engine off. If you can get a hold of a vacuum gauge and place it between the booster inlet check valve and the booster it self, then the above test can be more accurately viewed. I have a brass T piece in my tool box that I have used over the past forty years to plug between the booster a check valve, its come in bloody handy over the years for just this test.

[Dawso]

Thanks I'll give it a go.