[allan5oh]

It seems as though the brake action changes each time I hit the pedal. The application gauge will read the same, but the braking force changes. The foot valve is about 3 years old or so. I checked the pressure on the blue line and it matched the application gauge.

Haven't bobtailed enough to get a feel whether it's the trailer or truck. I do know pulling the spike say to 15 psi going 60 mph I can barely feel anything, even empty. Brakes are all in adjustment and there isn't any apparent foundation brake problems. S-cams, bushings, slacks, brake chambers all seem fine.

Trailer brakes were done a little over a year ago. What's the next step? Start pulling off service lines at the chambers to see what kind of pressure I'm getting?

[Windwalker]

You might try a little brake-line anti-freeze.

[allan5oh]

It's well above freezing here. Air dryer gets serviced once a year.

[YerDaddy]

The only thing that belongs in the air system is air!

You have ABS?

Might be your proportioning valve for the steer axle. Your problem is bobtailing right? The front brakes do most of the stopping then.

Having a quiet place to work enables you to listen (truck shut off) when brakes are applied to the air flow and brake movements.

[allan5oh]

My problem is apparent with the trailer hooked up, haven't done much bobtailing. Will probably get at that tomorrow. Both truck and trailer have ABS. I was originally thinking a relay valve somewhere, but I'll add proportioning valve to the mix. The trailer has one of those "all in one" relay/ABS valves for the service side.

Like I said before, pressure out of the blue line seems to consistently match the application gauge. I'm hoping to find service pressure somewhere that doesn't.

[LBF]

try the document library at Bendix

Bendix Commercial Vehicle Systems - Document Library

put bobtail valves into the search function, that might be where the inconsistent pressures might be coming from.

Do you have a schematic of your actual truck's system, in order to choose the right data sheets?

Get your pressure gauge hooked up at a chamber and do a bunch of applications to see if you get consistency, and that you get differences between trailer charged, and trailer exhausted, at the correct axle on the tractor for which bobtail valving you have.

Some act on the front, some act on the rears.

[allan5oh]

Originally Posted by LBF

Do you have a schematic of your actual truck's system, in order to choose the right data sheets?

I tried, but this happened:

http://www.classadrivers.com/forum/o...sed-volvo.html

[allan5oh]

It is a BPR-1 which is half way changed, after I get compression fittings. It has two quick release valves beside it. No wonder they just combined all the valves on later trucks. Dumbest spot too, right under the kingpin! Grease absolutely everywhere.

We shall see tomorrow if it is in fact the problem, but I don't think it is. I'll probably have to do the big valve on the trailer. Pretty sure it's more the trailer. This is the unit:

http://www.haldex.com/en/North-Ameri...BP-R1-/65145RX

[allan5oh]

Noticed a definite improvement, but my trailer is definitely not acting correctly. Pulled it then pulled another new trailer there was a huge difference. My dad said he could hear grinding sounds as I was applying the trailer brakes, maybe the hardware is worn out. All I know is the other trailers brakes worked many times better.

[allan5oh]

Changed both valves on the trailer (park release/trailer supply then the service valve body) and each time I changed one there was a noticeable difference.

However I noticed my drums are extremely worn, and they're only 15 months old. They started life at 14/16ths and now they're at 6-7/16ths. That's 75% worn! I'm very easy on brakes. I'll bet they have maybe 140,000 miles on them.

The brand they used is Gorilla Brakes.