TiredIron76

Considering the age of the truck, you might have two problems here; weak maxi springs could account for your poor parking brakes and a bad air supply to your service brake relay (R12) valve (or a bad R12 valve) could result in poor service brake performance. If your maxis are OEM and have never been replaced, trust me, it's time to junk them. Even if the springs are in one piece, they've already cycled about a million times and they're weak. You may have already done what follows, but if you haven't, it's worth a try: The R12 valve supply should come direct off the primary air tank. Drain the tank, undo the line from the valve, air the tank with shop air and you should get a blast of air from the line. If you don't, or if the air flow is weak, pull the line from the tank and check for sludge, collapsed or deteriorated line or any obstruction. Reconnect the supply line and undo the service (signal) line from the treadle valve. Have someone step on the treadle valve. The harder the treadle is pushed, the more air you should get from this line. The air pressure to your brake chambers is directly proportional (after valve crack pressure, no more than one and a half or two pounds) to the air delivered to the relay valve from the treadle. If there's plenty of signal air, hook it back up and pull a line at one of the chamber delivery ports. A weak airflow there indicates a bad R12. Replace that, and you may have it fixed. Assuming you've checked and corrected all the stuff mentioned in previous posts and that the basics (drums, shoes, brake hardware) are all present and correct you should have brakes on the truck. Good luck!

Sorry, bad edit. see next post

Back to diagnosing brake problems on Merric's 76 RD. First thing, Alex, I checked for air leaks from the maxibrake chambers. I've got an electronic stethoscope that can hear a fly fart at a hundred paces, so I blocked the wheels, aired everthing up and released the maxis. Not a peep, and believe me, the tiniest air leak sounds like a hurricane when I crank the gain up on this rig. Then to Keith's suggestion re TP valve. The trailer this truck pulls is a 20 ton tandem equipment trailer, pintle hitch, dates from the early 80s, no spring brakes. It has the old style emergency/parking system - when the supply air drops below 75 psig, it traps the air in the trailer and applies the service brakes. The trailer was completely rebuilt two years ago. We stripped off the deck, cut out the old crossmembers, tipped it on its side and sandblasted every inch, welded in all new steel, painted and new 2 1/2" oak deck. It was completely rewired and got a spindle- out brake job. Everything works. We aired the trailer, ran the guage pressure up to 120 and I popped the supply gladhand. Pressure was still over 90 when the TP valve cycled and shut off the air. So good news is, the TP valve is good. Bad news, I still don't know where the air is coming from at the service gladhand, trailer air shut off and treadle valve applied. It's not much - at moderate foot valve application, I can stop the air flow at the gladhand with my thumb. But it's there, and the truck would fail a roadside inspection. There's a dummy on the gladhand, so it's not losing air, but it ain't quite legit. I think my next step is to check the dash valves for leakes and bypassing. This truck has the old three valve system from the first FMSS 121 days, tractor park, trailer park and system park (part of the system designed by geniuses with too much time on their hands and no grasp at all of the real world of trucking). The 121 air control alves under the truck had been replaced when Merric bought the truck, so I'm dealing with conventional air flow there. Everything at the dash seems to work fine, but ya never know, might find something. I'm eventually (key word there) going to find the problem with this thing, and any advice from anyone out there that speeds up the process will be greatly appreciated. I live in hope. Thanks, Mark

This check valve was located on the primary tank port that supplies the service relay valve (R12) for the drive axles. There is no other component supplied by this port. I missed it completely when I first checked the R12 valve, which, in spite of its age, would probably have continued to function if the check valve hadn't disintegrated and plugged itself up. I have never seen a check valve in this location and I am totally baffled as to why it was put there. The other brake circuit (steer axle and trailer) is fed from the secondary, which in this truck is the front half of the tank. There is an internal check valve in the bulkhead between the two sections and if that goes, you junk the tank. All will become clear in the fullness of time. I have always been able to solve these problems, often with help and advice from friends. It just seems as if it takes longer lately. So it goes.

I'm going to check both (TP and maxi leaks) tomorrow if it cools off. Heat index of 96 and humidity around 110%. Time to move north I guess. Keith, you're sure right about lots of good info. I've been prowling around the site and seen everything from ACs to brand new Macks. If I had a nickle for every mile I've driven those old B models.... I'm not that tall, and when a guy my size is cramped in a cab, that cab is too damn small. There's a young feller down the road from me who's restored a B61, about a '51 or '52, which means it's 25 years older than he is. Truck looks like it just rolled out of the factory, and he works it about every day, hauls an honest eight yards and doesn't strain doing it. Alex, thanks for the tip. If I get back into it, I may start remembering all this stuff. It's hell gettin' old, but consider the alternative.... Thanks for the help, and I'll post the results. Mark

Truck is a tandem dump that started out in life as a tractor. Didn't have brakes enough to stop a baby carriage. Clutch was recently replaced, which necessitated removal of air tank (primary and secondary in one unit and there was some concern that the lines had been improperly re-installed, since the problem surfaced when it went back on the road. I started at the R12 valve, found plenty of signal air and what I thought was plenty of supply air. Found barely enough air at the ports to the chambers to move the slacks. Ha! bad R12 valve, right? The thing looked like OEM, so after a Saturday morning trip to get the only replacement available locally, replaced the valve, aired the truck and nada, nichevo, same damn thing, hardly any air to chambers. I drained the air, unhooked the supply line to the valve and this time I was not so happy with the volume or pressure from the primary. Maybe a bad line? When I looked at the back of the tank, I found a check valve that, if not OEM, had been on the truck for a LONG time. When I pulled the line off, the guts of the check valve fell out. I couldn't find a check valve in that position in any of my air schematics and couldn't figure any reason for it to be there. Removed it, blew the line out, ran it back to the R12 and got brakes that will put you through the windshield. End of first problem, I hope. Question: Has anybody ever seen a check valve in that position (right at primary tank port to feed relay valve)? The flow is tank > valve. What purpose would it serve? Problem #2: When service brakes are applied, air leaks out of the service gladhand. The guy who runs this truck is a very conservative driver; he works locally on the back roads, never gets up much speed and uses the foundation brakes to help the Jake. Before the service brake problem surfaced, I think he had plenty of air for what brakes he needed and didn't notice the leak (this is one LOUD old RD). Can anyone suggest a first place to check? I am leaning toward the TP valve, but after my first failure to diagnose, I don't want to buy any more parts just on a maybe. I used to do this stuff for a living, but it's been a lot of years and besides, I detest parts throwing. Any help will be sincerely appreciated. Thanks, Mark