I dream of rain

The root cause of the brake failures..

This is the electro-hydraulic actuator that controls my truck's rear-wheel anti-lock (RWAL/RWABS) brakes. To make a long explanation short, the actuators went bad, probably due to nineteen years of accumulated sludge and gunk in the brake lines.

There are two solenoids; one (normally open) isolates the system from the master cylinder (ensuring no additional pressure can be appllied), and the other (normally closed) lets the fluid into a bypass accumulator to relieve pressure on the rear brakes. The two cycle in turn as long as necessary so that the rear brakes rapidly pulse, ensuring the rear end doesn't slide out.

It turns out the bypass solenoid was stuck so it wouldn't return closed properly, and even the isolation solenoid was a little sticky. This was a recipie for very scary situations.

Due to the proportioning valve in the system, the front wheels don't engage until the rear wheel pressure is above a certain threshold. Since the bypass valve wasn't seated, the accumulator acted as an expansion chamber, causing the rear brakes to require a much larger volume of fluid before they'd begin to engage, finally allowing the front brakes to begin engaging as well.

Unfortunately, this didn't happen until the brake pedal had almost reached the floor, providing very little braking power when the pedal finally bottomed out. Annoyingly (and quite dangerously!) as the ABS controller occasionally cycled the system, it would randomly trigger or clear the failure condition depending on which valve was actuated, the pressure involved, the phase of the moon, and a critical D20 roll.

So. Last night I put the new part on, but I still need to bleed out the air I just re-introduced into the brake system. This will have to wait until after the new year -- Tomorrow morning I'm hopping on a plane for a new year's lark to Dubai.

And on that note, see y'all in 2014!