Pressure Protection Valve

[msksann]

Last week I took my Bird on a short 2-day shakedown run just 50 miles
outside of San Diego, far enough to reach operating temps. On arrival
at the campground, I heard a slight air leak, and crawling between the
Tag Axle and the Drive Axle, I found a device leaking air out of a
seal clamped down by 4 screws. The Pressure Protection Valve looks, to
me, like a small regulator similar to a propane regulator. Midland
part #N15759C (N157590) BB part #0522508. This has been replaced by
the Haldex KN31000, available at napa for aprox $38. Or online for less:
http://tinyurl.com/24ngm4
http://tinyurl.com/2zs6qn

Tightening the 4 screws about 1/8 of a turn stopped the leak, and
after crawling out from between the Axles, I fired up and waited until
the air drier purged at 120 psi, shut down, and crawled back under to
re-check the leak and surrounding area with soapy water, and then with
my ultrasonic leak detector. All good. Crawling back out, my
girlfriend, whose hearing is clearly better than mine is, told me she
still heard a hissing. Back underneath, everything still checked out
okay, but I did hear a leak further forward, so lying on my back, I
managed to pull my skinny body under the Drive Axle into into the next
forward compartment. This is the tightest, and scariest, place I have
contorted myself into yet! Sure enough, next to the air drier, and
loomed into a complex series of connectors, adapters, and other
unknown devices, was another Pressure Protection Valve, leaking thru
the seal just like the other PPV. Even I could not fit into this area,
right side (curb side of the air drier), but with the rear end yoke
pressing against my chest, and my left cheek (facial) resting against
the drier, looking out of the corner of my eye, I could just touch the
PPV with my fingertips. Okay, with much verbal lubrication, I managed
to tighten these screws as I did the other. Getting myself out of that
chamber as fast as I could, I re-pressurized the air system until
purge again, but this time, the air leak was even worse, and leaking
out of a small breather hole in the cap. Research time.
The PPV, with one inlet, 1 outlet, and 2 accessory connections, is
commonly used to isolate leaking air accessories from the air system.
Used as it is between the air tank, and the Ridewell air spring
suspension, only the inlet and outlet is used, with the accessory
ports plugged. I believe(?) this is what keeps the Ridwell air spring
suspension (airbags) from inflating until 65psi is reached, and closes
off if air pressure drops to 45psi. Back underneath, I managed to
remove the 4 screws, revealing a flat diaphragm, which clamped between
the two halves served as the seal, and a spring with a tiny metal cap
on the end to protect the diaphragm. Examination showed the diaphragm
was stiff with cracks throughout. Toadless, I had no problem having
NAPA deliver one to me at the campground, but being that there was no
way I could remove and replace the whole unit in it's location, it
seemed "easy" enough to just harvest the diaphragm out of a
replacement, and rebuild the original. Well, the newer replacement is
designed slightly different, with a smaller diaphragm counterset in
the center, instead of clamped
Hmmm, the diaphragm material is .05 inches thick, and I have seen this
material before used in a Par Water Pump rebuild kit I had on a
sailboat, but is this material good to be used with air and oil? Best
research I could do shows that it is (??) A call to a friend,
confirmed the same .05" thickness, and he delivered one out to me.
With twisted spine, more verbal lube, much complaining, (and a bit of
claustrophobia), I replaced the seal/ diaphragm, and it is working
properly. I suspect all of the material in these valves are getting
hard and forming cracks, and I plan on replacing them all with new
Haldex PPV's at first opportunity. The Tag Axle PPV can be done with a
little effort, and I will also have a shop replace the Drive and Front
Axle Suspension one that I rebuilt, (to alleviate lingering worry
about the material replacement).
On the air system diagram in my blue box owners manual, (Figure 10-13.
Air System Sheet 1), they are listed as part #2. I would be interested
in any feedback the many, more experienced owners on the forum might
have regarding failure. Am I am correct in the full purpose of there
valves as used in the Ridewell air spring system?

Thanks in advance!

Michael Kramer
86 PT-40 8V92
San Diego, CA