The Stovebolt Site Forums The Parking Lot Big Bolts Bleeding a Bendix Hydrovac equipped brake system.

Does the engine need to be running when bleeding the brake system on a '56 C6500 equipped with a bendix hydrovac system since the master cylinder pushes fluid into the hydrovac?
I recently replaced my master cylinder and some brake lines. The brakes felt hard after I bled them without the engine running but when I drove the truck I had to pump the heck out them to get them to work.

Engine can be running but vacuum should be disconnected first. Google 'Precision Rebuilders' ,look at their catalogue and manual, print out appropriate pages.

Ed

http://precisionrebuilders.com/hydrovac.html

From that page:

Install Booster
• Connect all lines making sure all fittings are clean.
• Bleed hydraulic system using a pressure bleeder.
• Do not run engine while bleeding hydraulic system. (See "Bleeding Procedures")

http://precisionrebuilders.com/ts_hydraulictrbl.html
Bleeding Procedures

I know very little about the Hydrovac, but I think I adjusted mine with the engine off (54/55 2-ton GMC).

I rebuilt the rear wheel cylinders in my 1958 6400, then tried to bleed and could never get a good pedal. After I rebuilt tha master cylinder, replaced all the lines (steel and rubber)and rebuilt the front wheel cylinders (pistons in the fronts were frozen) I bled it again and now have good brakes. (home-made power bleeder and engine off). All that said, make sure everything is in good shape or it may suck air in.

Also, make sure you adjust the top shoe on the rear brakes properly for bleeding (loosen it all the way up so it collapses the piston all the way into the wheel cylinder and keeps it from creating a pocket that can trap air.)

Thanks

I read in shop manual:
Bleed the slave cylinder in the hydrovac first
Then the upper bleeder of the hydrovac
Next comeslower brake cylinder furthest from hydrovac
Then upper of same wheel
Work your way closer to hydrovac lower cylinders first as applicable.

I did this on mine, starting with a dry master and hydrovac and even a couple of empty lines (replaced) and it worked like a charm.

A pressure bleeder will make life so much easier and will improve relations with your wife who may not like sitting in the hot/cold cab playing the up-down game with the brake pedal. But whatever you do, check the fluid level in the master cylinder OFTEN or you will have to start over. That little sucking sound from the master is such a downer.

I bought a pressure bleeder as specified/suggested in the Shop Manual.

As 1Ton_tommy posted, besides being the correct tool and bleeding procedure, it makes the job much easier.

http://www.speedwaymotors.com/Tech/pressure-bleeding-your-brakes.html

Motive pressure bleeder model 0102 for early GM

Try a search - I think other people have posted how they easily fabricated a pressure bleeder and master-cylinder cover/cap/connector.

I saw how several people built them from a pump sprayer. I am going borrow the wife's and add a female 1/8 air fitting at the air gauge to connect to a pancake compressor set to 50 psi.

My big concern is safety. Precisionbuilders recommends 50-60 psi and the Motive unit recommends 15 psi.

I will use a long air line with the tank sitting in a garbage can the first time to see if it will take 60 psi.