1995 W900L brakes hard in bobtail. HELP!

ok, i have a 1995 W900L converted into a quad axle dump truck.

There is a sticker on the dash that says the following;

Caution
Vehicle equipped with brake
proportioning system
increased brake pedal effort
may be required when braking
under bobtail conditions

It takes almost a full pedal push to get the back brakes to engage and when they do, it's right now!

The following is what we have done to try and fix and the problem was there prior to doing anything and here is what I know, .The foot valve is the same part number as a foot valve in a 1997 factory single maxi and a 2004 single maxi, so based on that, it isn't the foot valve.

I removed the dual maxi's and replaced with a single. orange and green as supply, yellow as delivery.

Removed the johnny bar hook ups..Plugged the supply line at the manifold in the dash.

Removed the 2 little silver valves from the back of the foot valve. Thought these were the proportioning valve mentioned in the little sticker on the dash.

Dealer doesn't have a clue, neither do any of the local ruck shops.

Hope someone on here has a clue..... Lol.

 

That ship has sailed.

There has to be valves or a valve that is regulating this, somewhere. about the only thing that is left as far as i can tell would be the brake valve right before the rear ends on the inside frame rail on drivers side (i think).

 

don't know off hand, will go check.

 

just called one of the drivers, he said 120 psi...... doesn't know what tank it is pulling from. i can get a better handle on that question tomorrow. I will give a follow up.

 

Yes, correct, now the question at hand is how does one eliminate that aspect?

the other thing is we keep breaking front brake drums.....