Thanks, Rick. I may take you up on that. It depends on what I find in the darkest depths of one of the storage boxcars buried under other stuff this coming weekend. I’ll let you know.

There has been a bit of confusion arise in this thread, so to help clarify things a little bit:

> Is the mounting bracket with the pipe connections the same for ABDW and AB for an easy swap-out or is that going to require a bit of plumbing adjustments to fit a new bracket?

>As far as the AB to ABD/W conversion, its just a matter of changing the top stud on the pipe bracket where the service portion mounts. It is shorter on the ABD/W and that's it.

Although the piping on the cars and the reservoirs are the same, the arrangement of ports on the pipe bracket faces are different. According to a friend who was a supervisor at a WABCo repair shop, you cannot bolt ABD valve portions to an AB bracket (and vice versa). Comparing the diagrammatic drawings of the two valves shows their service portion faces have a differing number of ports

That pin is a rejection device, something which WABCo has used in one form or another for at least 130 years to forestall improper assembly. The slide valve of an F-46 type triple (c1890) is machined so you can't install it backwards; for any cover which uses four bolts, the bolt pattern is trapezoidal, not square; for three bolts, they are not arranged in an equilateral triangle; and so on.

Modifying a rejection device is just asking for problems.

> If the latter, this likely indicates an issue with the control valve or related piping, being as the control valve itself is going into release position,

Not necessarily because internal components of the service and/or emergency portion could be leaking to atmosphere while in the lap position, such as the diaphragm in the limiting valve having ruptured or, more likely, the service slide valve leaking between the BC and EX ports.

> Is it bleeding off through the retainer (aka via the exhaust) ? Is so the automatic brake valve is releasing... for one of several reasons...

A number of people have discussed the problem as the caboose "releasing", which has created some confusion. Less confusion will result with the phrase "bleeding off" or "leaking off", if only because the automatic brake valve in the locomotive hasn't been used to release the train's brakes.

Being in the locomotive, the only way the automatic brake valve could affect the situation is by leaking air from either the main reservoir or feed valve into the brake pipe while it is in the lap position, in which case it wouldn't be just the caboose which is "releasing". If just the caboose, it's a problem within the caboose's AB valve.

> this amount of BP reduction should create about 50psi in the brake cylinder,

The amount of full service pressure developed in a brake cylinder is 5/7 of the brake pipe operating pressure being used. This principle goes back to at least 1895. However, when you start looking into relayed brake systems, cylinder pressures can be found to be violating this principle.

> Why would a caboose even have a retainer?

Depends on what the operating RR wanted.

According to my WABCo friend, FRA/CFR/AAR standards specify that a car to hold a brake application for a minimum number of minutes (5 or 10, forget which at the moment). This number should not be confused with the brake pipe leakage limitation standard of not more than 5 psi/minute, which goes back at least 110 years.

It's been my experience (some 50 years worth) that if an AB control valve sits unused for a prolonged period of time, it tends to operate rather sluggishly. F-46, H, K, and ABDW don't seem to be prone to this problem.

MT4351, on a couple of your comments:

I agree these leaks specifically called out (at least the best I can remember the port-to-port details of the AB off the top of my head) would allow a reduction of brake cylinder pressure while the valve itself would remain in lap position. Nevertheless, the fact that the symptom of air leakage at BC exhaust when in lap would indicate an issue with the control valve remains - being as the control valve is supposed to form the barrier between the cylinder and exhaust to atmosphere when in lap (or emergency) position.


Put a different way, standard pressure ratio since the dawn of air brake time is supposed to be 2.5psi of brake cylinder pressure for every 1psi of brake pipe reduction (until equalization is reached), per textbook perfection. But yes, there are many real world variables that can change this value (exact reservoir volume, exact cylinder volume (piston travel), valve performance, ect.). Relay valve operated systems may add yet another variable to full service/emergency pressure, as you point out.


CFR Part 232.205(c)(4) states: "...and the brakes shall remain applied until a release is initiated after a period which is no less than three minutes." This may be the passage you're thinking of. If there is some other "in the field"/"on the car" requirement than what is called out in the CFR or a Single Car Test Code, I'm not aware of it.

-Erich Armpriester

> Put a different way, standard pressure ratio since the dawn of air brake time is supposed to be 2.5psi of brake cylinder pressure for every 1psi of brake pipe reduction (until equalization is reached), per textbook perfection.

The ratio is 2:5:7 for a full service application, which translates to "take 2/7 of the operating pressure out of the brake pipe, get 5/7 of that pressure in the brake cylinder". This was developed for a 10" cylinder with a certain size auxiliary reservoir (1588 cu in). Increasing or decreasing the size of the BC (to 8", 12", 14", or 16") requires adjusting the volume of the auxiliary reservoir accordingly to maintain the ratio.

The ratio is slightly different for a quick service application because of brake cylinder displacement.

Although described by the universal gas law, I try to avoid the use of the term "equalization" when discussing air brakes because it becomes to easily confused with the equalizing piston and it's operation. It's a similar deal with the #6 distributing valve and "pressure maintaining", which WABCo says it has -- for brake cylinder pressures on the locomotive only. Most people, railroaders and non-railroaders alike, equate "pressure maintaining" with the feature which keeps brake pipe pressure to a constant value after the E has made a brake application, and this is an automatic brake valve function.

> But yes, there are many real world variables that can change this value (exact reservoir volume, exact cylinder volume (piston travel), valve performance, ect.).

In the latter 1890s, the MCB Assn specified backwards compatibility for new systems. This means that ABDX operates on this same pressure ratio.

> Relay valve operated systems may add yet another variable to full service/emergency pressure, as you point out.

The operation of the relay valve uses the same ratio, using a dummy auxiliary reservoir and dummy brake cylinder. What the relay valve produces in response when supplying the car's brake cylinder(s) is another matter (and it doesn't use the dummy auxiliary reservoir to supply the cylinders).

That’s exactly what I needed to know. So If I want to update the valves, I need to change the mounting bracket also. Can I assume that the current plumbing arrangement to the “new” bracket will remain the same or will I get into doing some piping modifications also?

We’re going to either have both portions of the existing AB rebuilt or update to ABD/W and fully service the cylinder. That way we know all is good.

To reiterate, it’s either the service portion or the cylinder that’s leaking off and releasing the brakes on the caboose only. It doesn’t seem to be leaking off through the retainer that I can tell so it’s either the AB service internally or the cylinder. The cost of the rebuild or exchange is low enough to just do it and be done. At least that’s my plan….

> That’s exactly what I needed to know. So If I want to update the valves, I need to change the mounting bracket also.

Yes

>Can I assume that the current plumbing arrangement to the “new” bracket will remain the same or will I get into doing some piping modifications also?

Possibly, I'm not sure.

> We’re going to either have both portions of the existing AB rebuilt or update to ABD/W and fully service the cylinder. That way we know all is good.

If you're trying to be a museum, updating is a no-no. Besides, ABD-W is obsolete.