Brake "failure" oddity that has me scratching my head.

At the track day last week the brakes were fantastic when first applied coming up to the corner. If I did a quick second application of the brakes there seemed to be little to no deceleration going on. The pedal was high and firm at both the first and second application. It just felt like a typical power brake system that has no vacuum to the booster. The throttle was closed through all this, so the booster should be getting vacuum to the front side of the diaphram. So, I'm confused on what's going on here. Booster is an '84 C4 unit.

Another possibility I was trying to figure out if the master cylinder was the culprit. It was hot to to the touch, and slightly overfilled. I drained out some fluid to reduce the vertical pressure column under braking, just in case the cylinder "ram" couldn't release the fluid quick enough under braking. (Far fetched, but I was really scratching my head trying to figure out the cause). I also tie strapped a length of foam tubing insulation onto the master cylinder to reduce the heat conduction, but didn't see any improvements. The master cylinder is a C5 aluminum part.

Another item I'm addressing is the vacuum tubing between the booster and the carb. I changed the vacuum plumbing over the winter, and for all I know I might have possibly caused this problem. I have two sources of vacuum (the carb base at the secondaries, and a manifold fitting right at the #8 cylinder runner). I also have two main requirements for vacuum: The brake booster and the PCV vailve. I used to plumb them to the two vacuum ports I mentioned, but I always hated that the port above #8 runner either injested oily fumes from the PCV valve, or injested raw air from the booster that leaned out #8 cylinder. So, over the winter I hooked the booster hose and the PCV hose together at a tee about 1.5" behind the carb base fitting. I figured that I shouldn't get a noticeable pressure rise (vacuum loss) over that short of a distance. But, I can be wrong, and maybe that was the cause of the vacuum booster acting odd. I don't know. Either way, I am now making a way to pull vacuum straight from the manifold plenum, and I will hook the PCV hose to that, and the card vacuum post will hook to the booster.

That;s where I'm at. I welcome comments and advice on this issue.

69427 said:

Another item I'm addressing is the vacuum tubing between the booster and the carb. I changed the vacuum plumbing over the winter, and for all I know I might have possibly caused this problem. I have two sources of vacuum (the carb base at the secondaries, and a manifold fitting right at the #8 cylinder runner). I also have two main requirements for vacuum: The brake booster and the PCV vailve. I used to plumb them to the two vacuum ports I mentioned, but I always hated that the port above #8 runner either injested oily fumes from the PCV valve, or injested raw air from the booster that leaned out #8 cylinder. So, over the winter I hooked the booster hose and the PCV hose together at a tee about 1.5" behind the carb base fitting. I figured that I shouldn't get a noticeable pressure rise (vacuum loss) over that short of a distance. But, I can be wrong, and maybe that was the cause of the vacuum booster acting odd. I don't know. Either way, I am now making a way to pull vacuum straight from the manifold plenum, and I will hook the PCV hose to that, and the card vacuum post will hook to the booster.

Click to expand...

I'm just now chasing Vacuum Leaks in the engine bay on the 2008 Chevy 2500 (tow) Van-Camper [6.0L LS uhuhuh ].

There is a Check Valve (apparently -- yet to disassemble) in that hose from the vacuum source to the brake booster. Once I reassemble everything, I can check my booster for the leaks.

The method I see is starting the engine, pressing the brake pedal to the floor, then turn off, and hold down 30 seconds. If the pedal rises (with light pressure still on it - its the booster. - So says YT. Then I'll check the gasket between the MC and the booster.

Again, my problem is slightly different - but in the same "neck of the woods." If the check valve is bad - could that be creating your problem? - Can't say.
LINK if helpful:
Good Luck,

Cheers - Jim

I have never checked but is it normal for the brake cylinder to be hot to the touch?

Sounds like the crankcase vapor is getting into the booster (somehow)? Maybe consider plumbing the PCV into the air filter housing like older GM cars?

Boiling your brake fluid?

I put a vacuum reservoir on my car and it helped when car went to idle.

BBShark said:

I have never checked but is it normal for the brake cylinder to be hot to the touch?

Sounds like the crankcase vapor is getting into the booster (somehow)? Maybe consider plumbing the PCV into the air filter housing like older GM cars?

Click to expand...

I'll confess I've never had any cause to put my hand on the master cylinder before, so I don't know if it's a common occurrence or not with my car. It was an 80* day, and I had spent about 15 minutes out on the track, so the engine and coolant temps were about 210*. I have some aluminum sheets at the frame level to dissuade air from exiting the engine compartment via the bottom of the car, and instead entice it to leave via the side gills. I suspect that the master cylinder in enveloped in hot air during all this.

I'm trying to figure out if there's some way that I'm getting some "crosstalk" between the contents of the two vacuum lines, but I haven't arrived at an answer yet. Part of the unknown is if the checkvalve at the booster ends up allowing slight pressure back into the line once the checkvalve closes. I just don't know.

So for now, I'm shotgunning things. I replumbed the line from the booster to be the only line attached to the carburetor rear vacuum nipple, so there can't be any interaction with any other vacuum line. I'm also working on being able to hook the pcv valve line to the intake plenum, so that any oil fumes or combustion blowby will be equally distributed/diluted to the cylinders, rather than it all ending up going into the vacuum nipple right in the port feeding cylinder #8. And lastly, I'll be flushing the system with new brake fluid just to reduce the list of possible contributing factors.

If it hadn't been for this odd brake issue, it would have been an absolutely perfect track day.

rtj said:

Boiling your brake fluid?

I put a vacuum reservoir on my car and it helped when car went to idle.

Click to expand...

I don't believe the fluid was boiling, as I had a firm high pedal throughout all this, but I'm putting fresh fluid in it this week anyway just to give me a bit more peace of mind.

I'll keep the vacuum reservoir option in mind.

phantomjock said:

I'm just now chasing Vacuum Leaks in the engine bay on the 2008 Chevy 2500 (tow) Van-Camper [6.0L LS uhuhuh ].

There is a Check Valve (apparently -- yet to disassemble) in that hose from the vacuum source to the brake booster. Once I reassemble everything, I can check my booster for the leaks.

.........................

Again, my problem is slightly different - but in the same "neck of the woods." If the check valve is bad - could that be creating your problem? - Can't say.
.....................

Good Luck,

Cheers - Jim

Click to expand...

I am definitely going to look at the checkvalve. My booster is an '84 Corvette unit (small and light).

I would try the air filter housing vent filter for the PCV. Not like you have to have a good vacuum source to vent the PCV.

BBShark said:

I would try the air filter housing vent filter for the PCV. Not like you have to have a good vacuum source to vent the PCV.

View attachment 56884

Click to expand...

The factory setup on my car has that large size hose going from the right side valve cover up to the air filter base, and the PCV valve draws out of the drivers side valve cover. I've reworked/relocated the PCV valve vacuum source to a couple threaded holes that enter the plenum of the (Edelbrock C396) manifold. Also, the power brake booster is now getting its (unshared) vacuum signal from the nipple under the carburetor rear fuel bowl. The booster should be getting a guaranteed clean high flow vacuum now. The only thing I haven't tested yet is the booster vacuum check valve. I'm still trying to figure out how I'm going to do that.

One of these?

Mine does pressure or vacuum. Pull the hose (PITA) then just use one end - suck/blow. Then you'll know.
My problem - can't see th ehose on the booster to pull it off!
Best of Luck..

Cheers - Jim

phantomjock said:

One of these?
View attachment 56886
Mine does pressure or vacuum. Pull the hose (PITA) then just use one end - suck/blow. Then you'll know.
My problem - can't see th ehose on the booster to pull it off!
Best of Luck..

Cheers - Jim

Click to expand...

Thanks, yes I recently bought one of those. I spent some time today thinking about the booster operation, and I have a couple ideas on how to check a few aspects of the high and low pressure sides of the booster. I'll be trying to get that done this week.

I'm with 'boiling the fluid' camp. My FJ40 initially would have this problem. The hydraulic clutch would work great when cool but because of where it was at, the cylinder got very hot - and made pushing the clutch quite hard. Also, a dragging brake or brakes will do this. what fluid is in it? the worst part about a dragging brake is the problem just gets worse - the more it drags, the more the fluid expands, the more the fluid expands, the less braking you have available.

vacuum systems either work of they don't - the only exception to that rule is if there isn't a check valve. From what you describe, the issue would be more pronounced at idle.

Lots of us used silicone brake fluid due to the oem cast iron brakes. Any chance you are using that?

Last post addresses issues.

rtj said:

Lots of us used silicone brake fluid due to the oem cast iron brakes. Any chance you are using that?

Last post addresses issues.

Silicone Brake Fluid

I have a bumper-pull trailer with hydraulic drum brakes. I have redone three of the wheel cylinders today, and replaced the master cylinder with a new one (I'll rebuild the old MC and keep for spare) The system is presently empty of brake fluid. This would be a chance to go to DOT 5...

www.practicalmachinist.com

Click to expand...

rumor has it silicone brake fluid does not play nice with "new" rubber seals, I got some calipers from Lonestar a week or so ago with a disclaimer on each box, "warranty void if you use silicone fluid".

rtj said:

Lots of us used silicone brake fluid due to the oem cast iron brakes. Any chance you are using that?

Last post addresses issues.

Silicone Brake Fluid

I have a bumper-pull trailer with hydraulic drum brakes. I have redone three of the wheel cylinders today, and replaced the master cylinder with a new one (I'll rebuild the old MC and keep for spare) The system is presently empty of brake fluid. This would be a chance to go to DOT 5...

www.practicalmachinist.com

Click to expand...

No, I've never put silicone fluid in any of my cars.

I like your idea if re-routing the vacuum lines; you changed 'em this winter....now you have the symptom. I agree boiling brake fluid isn't the problem. I agree getting a gauge on it should help you see what's going on. A well functioning booster should give you about three brake pumps with full power assist.....so even if your vacuum source were marginal, I'd think that it'd give you two good brake applications for each brake event. I guess I'd vacuum test the booster itself and see if it's leaking.

That IS an odd one....I'm looking forward to hearing more about this as you dig into it.

Dual Brake Gauges with video capture of the "event" would be useful post session. Might indicate a loss at either end - if both drop then the booster.
No indication of fluid anywhere?
Getting close to sounding like the master cylinder...

Cheers - Jim

Next track day is in a week, so I'm trying to take the car for a test drive around the neighborhood and up the interstate, but can't move the car until I get a stinking cotter pin to hold the RF caliper pads in. The Wilwood calipers use a three and a half inch long skinny cotter pin that seems to be unavailable from any source other than Wilwood and their dealers, and my order seems to be taking the long scenic route getting to me. In the meantime I'm trying to figure out a safe substitute for that pin so I can at least drive the car a short bit to see if I've made progress on this issue.

Amazon shows the available overnight.

But, if they are truly 1/8 in, steel tig rod would probably work. 1/8 bolt, etc.