quote:

Originally posted by Rusty:
Carl, my system is entirely different from yours. Mine is Hydroboost (Hydraulic over hydraulic); yours is air throughout.

Hmmmmm My breakaway is air over hydraulic and has to be bled same as any hydraulic system. I usually do this once each year before I start out, I flush the entire system with new fluid to get rid of any water that may have accumulated over the winter of inactivity.

MW, I also have air over hydraulic and contrary to what I have read here I opted for silicone brake fluid and it will NOT hold any moisture... That may seem wrong to some people on this forum but I think it will save me the yearly maintence..
Hopefully...

The brake system on mine is fine - it's the hydroboost with the issues...

I'm really baffled where the aeration is coming from, and the ongoing symptoms.

I thought things may have stabilized, but something is draining fluid back into the P/S reservoir. Went up about an inch overnight. Letting it rest tomorrow, then explore more Monday. The only suspect left is the shaft brake system.

Started the bleed routine today, by disconnecting and plugging the brake booster return and attaching a clear hose to it which drains into a plastic jug. Four bleeding cycles, and I think I'm making progress. I still have air, but it seems like a tiny bit less each cycle. With around 100' of Hydroboost lines, this is going to take a while.

Surfing the Web has revealed:

1. The reservoir overflow issue is not uncommon, but NO ONE has posted a cause.

2. Self-bleeding the system by driving only works only "most" of the time. I'd agree; mine is a higher volume system than the pickups, and there may not be enough rest time for the fluid to deaerate.

3. On GM trucks, it has sometimes taken 6 or 7 cycles to get the system fully deaerated, and 1/2 to 1 hour sitting between cycles is recommended. (One owner had his in the shop for 2 days just for that).

On most hydraulic boost systems there is an accumulator that is either spring loaded or a pressure vessel we usually call the "bomb". it has a bladder that has a back fill of nitrogen. the purpose of these is to cause an immediate release of pressure to the braking system when the brakes are first applied to allow the hydraulic pump to catch up. It improves brake response greatly particularly in a "panic" situation.

On these systems, we generally shut off the engine and pump the brakes many times until we feel the accumulator has been depleted, all pressure is out and the fluid level in the accumulator is minimum. THEN we check the fluid level in the reservoir. In the manner, there is no fluid source to overflow! If the fluid is checked and topped off while the engine is running, then the accumulator is filled and later when the vehicle is just sitting (off), the pressure in the accumulator will force fluid back to the reservoir. When the system is new, there is no leakage but after aging, cold weather, seals do not prevent this from happening. The result is a reservoir overflow.

This is very common problem as you say BUT the root cause is well known.

On some vehicles if the vehicle is running, there is no fluid above the screen, probably down about 2 inches, once the engine is off and the brakes are repeatedly pumped until a change in pedal pressure is felt, the fluid would be right at the top. When people top off with the engine running, they wonder why there is fluid on the floor the next day, it is much worse in winter months as cold temperature effects the sealing issues and allows a lot of bleed back from the accumulator.

Thanks, Ed. There is an accumulator (of some sort) on the left frame rail; I had thought it was for the shaft brake. There is, however, an accumulator on this brake booster.

The aeration is still a puzzle, although as I go through the bleeding cycles, it seems to be moderating.

Sadly, there is precious little tech data about Hydroboost on the 'net.

Bleeding completed, I ran the rest of the tests and was able to identify the source of the problem: Shaft brake actuator. 

Glad you finally found it Rusty.... I'll bet you remember that one for awhile...

quote:

Originally posted by Patch1st:
I'll bet you remember that one for awhile...

I know I will!

Remember what? Oh, yeah, the thingamajig that goes on the watchacallit.

Anyway, After purging the air out of the system with engine running in "park" (brakes and steering worked fine, reservoir level was stable), I shifted into "D" then back to "P", then shut the engine off. The reservoir overflowed immediately.

The only component that is out of the loop in "P", but not anywhere else is the actuator.

On the aeration problem, replacing the brake actuator has solved the problem of the overflow.

After over 2 years of pusuing the source of the intermittent shimmy/vibrations at speeds over 58 mph, Occam's Razor has prevailed.

On my way home from the Hillsborough River State Park GTG, I stopped by Town Tire in Gainesville to get the front tires balanced. The service manager noticed the tires were wobbling on the balance machine.

Two new tires later, the shimmy appears to have been eliminated.

The front tires were replaced almost 3 years ago (blisters, Kellys) with Wanli (Chinese). Replacing old bad tires with new bad tires didn't work...

I found his site:
http://www.bendixbrakes.com/techCorner/faq.php
sponsored by Bendix. You might ask them about your hydro boost problems.

Thanks; the hydroboost issues are solved (assuming this year's 4th P/S pump doesn't break its shaft).

quote:

noticed the tires were wobbling on the balance machine.

Wheel alignments and static balancing never found the cause of the vibrations on my Corvair. A high speed spin on the Hunter machine showed two Pirellis to be out of round.

I had to show the dealer that when the car was jacked up, the tire was spun the quarter on the floor only flew away when the high spot got to the low point. I thought that was sufficiently graphic since we were spinning the wheel by hand but he still required explanation about why this happened. Part of the explanation was moving the high spot to the top and placing 3 quarters at the same point with the tire not touching.