The Stovebolt Site Forums The Parking Lot Big Bolts 57 6400, brakes & 2speed Axle Trouble

Hey everybody...

I just inherited a 57 6400 that has been sitting forever. Got the engine running and can move the rig around but I have no brakes and I cant get the 2 speed axle to go into high range. I was told by one of grandpa's brothers that the old truck had a vacume problem so I started diggin around and have seen some sort of vaccume thing connected to the master.. Just trying to figure out where to start.... Emergency brake works otherwise I would be in real trouble...


Help me please????

Mark in Alaska

Mark in Alaska,

I'm no expert, but I would think a '57 would have a electric two speed vs a vacuum type.

Eaton made a ton of them...a red knob affair attached to the shift lever! It is fused, and the motors do get 'sticky' after sitting around and not being cycled. Good luck

Stuart

Mark- I've got '57 5700 with 2 speed rear axle. Unfortunately I also inherited the brake and hi/lo shift problems. Fortunately I've got the problems mostly sorted out. I'd imagine your 2 speed axle has a diaphram on the rear axle that is activated when the knob is pulled by placing vacuum on the diaphram causing the axle to shift. Also the brakes are a hydrovac system. What you saw the vacuum hose going to was not the master cyclinder but rather a Bendix Hydrovac. Most likely you have a vacuum leak somewhere and also the hydrovac as well as the 2 speed rear shifting mechanism are probably stuck from years of nonuse and need some TLC to get working properly. To start it may not hurt to bleed the brakes, hyrdovac and mastercylinder to make sure you've got fluid everywhere. There is some info on the net and a lot of good archives here that have helped me.

Hey KSbee, Got any pictures of that '57 5700 LCF. I had a '55, & a'56 5700 series LCF(Low Cab Forward).

If your Grandfather know he had a vacuum problem he may have capped the line going to the Rear and Hydrovac in hopes of fixing it.

The rear 2-speed shift switch is bolted to the drivers side of the trans, just below the floor board. The switch is cable operated from the cab. There should be 4 vacuum lines on the switch, 1 for vacuum supply from the motor, 1 small line to the speedometer reduction unit on the back of the speedometer (keeps the speedometer calibrated for the two different ratios), 1 to the rear for low range and 1 to the rear for hi range. Check for vacuum at the switch.

There should be a "fresh air" filter on a large rubber line attached to the Hydrovac, make sure it's not plugged up. The filter could be behind the seat or on the inner fender under the hood. This filter allows air to flow freely back and forth on the non-vacuum side of the Hydrovac piston.

Diaphram for the rear shift unit are still available new...check with a BIG GMC dealer, they were used for many years.

Mike B

Wow everybody... Thanks for the starting point. The real bummer is that now it is BUTT cold outside with a 50mpg wind making the windchill about 40 below... I will have to wait till this stupid wind slows a bit so I dont freeze the brake fluid!!! Thanks again and I will let you know how it goes when the wind quits.

Mark in Alaska