Background: 1970 with power brakes. Entire system gone through 3+ years and about 5000 miles ago, new rotors, calipers reconditioned, not sure what if anything was done to the master cylinder.

Problem: most times braking occurs as normal. Occasionally, I get a feeling at the pedal like that the brakes "let go" for a split second, but then grab again normally. I have not been able to re-create it on demand despite several tries, it happens when it happens. The pedal does not drop noticably, but the driver's heart rate definitely skips a beat, it feels pretty scary.

Done so far: My neighbor and I tried to inspect the system for leaks, but couldn't get the car high enough for a good inspection. Took it to a local mechanic who went through the entire system and found nothing wrong. At my request, he took it for a test drive and did not feel anything abnormal. I went to his garage and inpsected the entire system on the lift myself, no visible brake fluid leakage was found. Back at my garage, I pulled each wheel one by one, removed the brake pads and checked the calipers closely for leakage or hang-ups, none found. One brake pad looked like it had a minimal amount of grease contamination, cleaned with solvent and re-installed. I was out for about a 25 mile ride on Saturday, and the problem happened twice.

Any thoughts?

 

The old test for master cylinder is to apply the brake and then hold that foot pressure steady for a period of time. The pressure will initially hold but release after a time. You will feel the pedal going down.

 

When the bad feeling happens, I don't notice the pedal dropping at all. Are you thinking bad master? Could it be something with the booster?

 

If the pedal doesn't move, this would be my guess. A momentary loss of power assist may reduce pressure on the brakes... I never heard of this happening, wonder if there may be a valve internal to the booster which leaks?

 

Make sure that the check valve in your vacuum line to the booster is working. Otherwise you might be feeling fluctuations in manifold vacuum.

 

my 2006 Silverado is doing the same thing,never had a brake system do any thing like it. Be nice to find out why.The brakes on my 69 vette are fine though.I'll change the m.c. on the truck and you change the booster on the vette and we'll find out which it is o/k?

 

Can't find anything mechanical, go to a complete fluid flush.

Get the old fluid out and replenish with new. Make sure to keep clean fluid in the master cylinder throughout the process.

 

This is never a bad idea, but the whole system was gone through 3 years ago, so it got all new fluid then.

I think CA-Legal-Vette has a good idea. I tightened up my vacuum system a year and some ago, but I know I still have some leaks. I have noticed the idle is a little rougher now than it was, maybe leakage is up again and messing with my booster. This weekend, I'm thinking of raising the idle a little and seeing if the problem gets better or goes away. If it does, I'll know what I need to do.

 

Not many people do it, but a fluid flush should be done every 2 years.
Don't know what you really mean by "brakes let go", but when master seals start to go south, they will intermittently not work under very light brake pressure, but still hold well under most braking. Just keep an eye on it.
Easy way to test the vacuum, is to cap everything off at the carb, but the dist canister and the booster, see if the idle improves.

 

Good stuff, thanks.