4700 Parking Brake

dmcantoo

New Member
I have a 4700 series Bobtail (2000) and the parking brake does not engage. What could be the problem and what do I need to do to fix it?
 

Racer X 69

Member
Gonna need a bit more information here.

Does this rig have air brakes or juice brakes?

If it is air, is it when you pull the yellow knob out the brakes won't set?

If it is hydraulic, then there should be a lever you pull on to set the parking brake. And if that is the case, is it that when you pull the lever the brakes won't set?
 

dmcantoo

New Member
This is a hydraulic system. When I pull the knob out to set the brake, it does not set. The truck can still roll.

At this time, even when you put it in "park" it still rolls. I just had the transmission replaced and that did not fix the problem.

So I am trying to get both issues fixed. If I can get the "parking brake" fixed, I'd be ok.
 

Racer X 69

Member
At this time, even when you put it in "park" it still rolls. I just had the transmission replaced and that did not fix the problem.
Park?

What brand of transmission does it have? Automatic truck transmissions do not have a park position.

And trucks with hydraulic brakes usually have a mechanical drum brake.

So what work have you been doing to the brakes?
 

mechman

truck mechanic
The switch could be bad, although usually then they won't release. Problem is usually with the solenoid/valve assembly, or wiring. These are no fun to work on, especially if you don't know what you're doing. When they get old, sometimes you end up replacing lots of parts. If you do get it going, take off the plastic pump reservoir, clean it out, and put in fresh ATF. Replace the plastic cap with the cap from a Briggs and Stratton small engine. No kidding. The original cap is not vented and water will suck into the reservoir past the moulding lines on the tank.
Of course this all assumes the shoes are good and the cable is adjusted.
 

Racer X 69

Member
He said it is a 4700 series. Those are medium duty rigs like this:

View attachment 13310

The ones I worked on at the power company all had air brakes, but they can be had with juice brakes.

Also, most of them had Allison automatics (a true auto, with torque converters and planetary gearing) and did not have a "park" position. The shifter was labeled R-N-D-3-2-1.

The older rigs I worked on with juice brakes had a hand brake that pulled a cable, which operated a drum brake mounted on the tailshaft of the tranny.

Some also had a MicoLock hydraulic line lock that locked the drive axle brakes and was clearly marked "Not For Parking". I never got a clear answer about the purpose of the MicoLock and they always bled off. Kinda useless I think.
View attachment 13310
 

dmcantoo

New Member
This is an Allison transmission. The shift lever says - PB. I have been told this activates the "parking brake", but is not actually in "park". My son-in-law has been trying to get a scamatic so he can do the work on it. But right now, nothing has been done on it.
 

Racer X 69

Member
This is an Allison transmission. The shift lever says - PB. I have been told this activates the "parking brake", but is not actually in "park". My son-in-law has been trying to get a scamatic so he can do the work on it. But right now, nothing has been done on it.
So you bought this truck in this condition then?

The reason I ask if any work has been done on it is that should provide clues as to what may be the malfunction.

One thing you can do is park the thing on level ground, block the wheels, and then place the selector in PB. then get under it and see if the cable that actuates the park brake mechanism is tight or loose.

That should give an indication if the cable is too loose or not, and if it is actually moving like it should.
 

mechman

truck mechanic
what racer x 69 said would be a good starting point. if the cable moves when it is put in park, the cable needs adjustment or the brake parts are worn out. It uses the same Bendix components as the one with the hand lever. If worse came to worse, you could probably convert it.
 

Hulio

New Member
I think ,this is the same thing that is going on with my truck .Good to know its not the transmission .Phew
 

Hulio

New Member
My Current report from the mechanic park brake not working at all, it s a bendix hydraulic spring actuated park brake, it s linked to the transmission with a 4 way valve and spring brake.
 
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