I brought one of Jim H's pressure bleeders to Bean Station where we bled the brakes on George Z's GMC after he installed the
reaction arm system he won at a GMCMI Convention.
We installed a clamp to hold the teat on the front of the combination valve in and then pressurized the bleeder to 10 psi.
We bled the calipers one at time in the following order:
1) Passenger side - Rear
2) Passenger side - Middle
3) Driver side - Rear
4) Driver side - Middle
5) Passenger side - Front
6) Driver side - Front
It took less than 20 minutes and we got ALL the air out of the system.
This scenario was repeated on JohnT's GMC earlier this year in Humble, Texas.
Regards,
Rob M.
The Pedantic Mechanic
The pressure bleeder is the way to go. But if I understand what Mr. Henderson wrote, the switch piston can completely shut off the flow of brake fluid to one side if the differential is great enough. Yes that is good to stem the loss of fluid if there is a leak, but it also means it can prevent you from pedal bleeding the brakes. I never encountered that before.
But if I had to conjecture how that could account for the pressures Neil has observed at assorted points in the line, that scenario would start with the premise that there is an air bubble in the rear. As the pedal is pressed, the bubble compresses and pressure builds to 500 PSI. BUT, at that point, the front pressure is higher enough that the "shuttle" has moved far enough to shut the rear line off from the rest of the system, and the pressure between the master cylinder shuttle continues upward to 800 PSI. So holding the pedal down, you got 800 PSI from the master cylinder to the combination valve and 500 PSI from the combination valve to the rear calipers. And it stays that way until you disturb the system.
Opening a rear caliper bleeder would leak it down, then the fluid would stop flowing, even while there was still 800 PSI at the master cylinder, and the pedal would not move at all.
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