On our last trip, On occasion, when I was applying the brakes very lightly, I could feel a very gentle pulsing in the brake pedal. It wasn’t a rapid pulse, like that when the anti-skid kicks in.
It was a very mild pulse, about once a second, and felt like the power assist was kicking in, then kick out, kick in, kick out.
After studying up on the power brake / power steering system, I suspected something in the Hydro-boost assembly was probably sticking or leaking and would probably need to be replaced. A re-manufactured unit is available from Rock Auto for about $110 (Cardone 527367).
There are two styles of Hydroboost unit. The difference is how they provide boost when the engine isn’t running. Electric and Accumulator. Mine uses the Accumulator version.
Prior to ordering a Hydro-boost assembly, I’d flush the system and put in fresh fluid, and see if this would free up any o-rings, etc. What the heck, I’d need to do this anyway if I install a new Hydro-boost assembly.
The Hydro-boost system (power brakes) gets its power from the power steering pump. It uses an accumulator to provide power assist in case the engine stalls. The regular brake master cylinder bolts to the Hydro-boost unit. You can see it in these photos. You can also see the accumulator, looks like a 2 inch diameter tube sticking out of the Hydro-boost unit (Mine is blue and you can just barely see it in theses photos.
I started looking at the hoses to figure out the best way to drain and flush the reservoir when I discovered that the hose that comes out the bottom of the reservoir and feeds the steering pump had a very serious kink in it. The kink was about 6 inches below the bottom of the reservoir, where the hose made a very sharp 90 degree turn and goes through the firewall.
You can’t see how bad the kink is in the above photo because there is a protective cover material over the hose.
However, after I removed the hose and the cover, I seen just how serious the kink was, and its quite possible it was restricting fluid flow to the pump, and causing the pulsating.
DRAINING THE OLD FLUID
I removed the 3/4 inch pump supply hose from the pump to completely drain the reservoir tank. This is the kinked hose that I had to replace.
I also removed the return hose that goes through the cooler to drain that part of the line. I left this hose disconnected to do a quick flush.
The coolers lower hose comes from the Hydro-boost unit. It goes through the cooler, and returns to the reservoir through the upper hose.
While the fluid was dripping out of the disconnected hose and cooler, I went inside the Mirada, and without the engine running, applied the brakes full and hard several times to drain the fluid trapped in the Hydro-boost accumulator. I did this 5 – 6 times.
The fluid I removed was a little dirty, I suspect it was probably original.
REPLACING THE KINKED HOSE
I bought a 4 ft length of 3/4 inch heater hose to replace the kinked hose. I routed the hose so there were no sharp bends and ended up using about 3 ft of hose.
FLUSHING SYSTEM
The power steering system calls for Automatic Transmission fluid. I use Valvoline ATF synthetic, the same stuff I use in the transmission. Its available from Walmart for $18 a gallon. I used the entire 1 gallon in this procedure.
I filled the reservoir with ATF and made sure the disconnected cooler hose was pointed into a container. I then started the Mirada and run for about two seconds. The cooler hose spewed fluid, and this flushed out the remaining old fluid from the pump, the Hydro-boost, and the cooler.
RE-FILLING SYSTEM
I then re-connected the cooler line and refilled the reservoir tank. I started the Mirada and cycled the steering back and forth a few times, and also applied the brakes a few times to ensure the systems were fully purged and bleed.
I then topped off the reservoir and went for a drive.
On my test drive, I didn’t get any pulsing in the brake pedal. I’m hoping the kinked hose was the problem. If not, I’ll replace the Hydro-boost cylinder assembly