Scubapro MK 5 1st-stage leak, probable cause and repair ?

[scubafanatic]

Hello,

I've acquired a few of these 1st-stages and had them serviced this summer, now I'm finally getting a chance to do underwater bubble leak tests in the tub, and have so far found one 1st-stage leaking bubbles from the 1st-stage ambient chamber holes. (this leaking one has the 4-hole non=SPEC ambient chamber) It's a subtle leak, too small (so far) to be detected via SPG/AI computer or audibly, but it's definitely visible underwater, emitting a 1/8" diameter bubble every approx 10-15 seconds from one of the 4 holes. The bubbles are coming from the LP swivel turret side of the 1st-stage. I don't think the leak is related to the turret, I suspect it's coming from the HP piston O-ring.

I know it's a minor leak, but it still bothers me, especially as it could be the precursor to a much bigger leak with usage/passage of time.

Hopefully it's just needing another new piston O-ring, and not that the piston or chamber is scored in any way.

IP starts @ 125 PSI, gradually creeps up to 135 PSI and locks @ 135 PSI. Tank pressure approx 3000 psi.

Would you agree in my probable diagnosis ?

Thanks in advance!


Update: OK I timed the bubble interval @ 1 bubble every 30 - 35 seconds.
​

 

[Tbar]

It could be either the Piston Shaft o-ring (high pressure) or the Piston o-ring (Intermediate Pressure). I have had the shaft o-ring fail on an MK25 and there was a lot of air leaking!

You might find that it is from the Piton o-ring as the 'cylinder' has not been well cleaned and the o-ring is not seating well.

 

[redacted]

You should be able to determine which o-ring is failing to seal. With the 1st under pressure, horizontal, and an ambient chamber hole at the top, get it leaking. Then rotate the turret end about 30 degrees upward keeping the ambient chamber hole at the top. If the leak is from the piston heat, it should take a while for the gas to pool before it starts leaking through the hole. If the leak is from the HP o-ring, it should continue leaking with little hesitation. Just to be sure, repeat, rotating the turret end downward. This time a hesitation would indicate an HP o-ring leak. uninterrupted flow indicates an LP o-ring leak. Repeat to confirm. Inspect the suspected o-ring and hope you can find a problem. Defective o-ring or nicked HP o-ring may be the problem.

I have found ambient chamber seating surfaces that were damaged (scratches and corrosion) and been successful repairing them with rub with a bit of brasso on my finger. I have also found pistons with a slight scratch on the shaft. You might be able to rub that out but I just swapped the piston with one from the junk yard (a $3 Mk7 with the light yoke). There are a lot of Mk5/7s in the junk yard that have plenty of useful parts

 

[scubafanatic]

You should be able to determine which o-ring is failing to seal. With the 1st under pressure, horizontal, and an ambient chamber hole at the top, get it leaking. Then rotate the turret end about 30 degrees upward keeping the ambient chamber hole at the top. If the leak is from the piston heat, it should take a while for the gas to pool before it starts leaking through the hole. If the leak is from the HP o-ring, it should continue leaking with little hesitation. Just to be sure, repeat, rotating the turret end downward. This time a hesitation would indicate an HP o-ring leak. uninterrupted flow indicates an LP o-ring leak. Repeat to confirm. Inspect the suspected o-ring and hope you can find a problem. Defective o-ring or nicked HP o-ring may be the problem.

I have found ambient chamber seating surfaces that were damaged (scratches and corrosion) and been successful repairing them with rub with a bit of brasso on my finger. I have also found pistons with a slight scratch on the shaft. You might be able to rub that out but I just swapped the piston with one from the junk yard (a $3 Mk7 with the light yoke). There are a lot of Mk5/7s in the junk yard that have plenty of useful parts

OK, I'll see if your procedure narrows down the problem, it's after sundown here now and the tub is outside where natural sunlight makes it easier to see leaks so I'll try it tomorrow and report back.

Thanks awap!

 

[halocline]

Hello,

The bubbles are coming from the LP swivel turret side of the 1st-stage.

IP starts @ 125 PSI, gradually creeps up to 135 PSI and locks @ 135 PSI. Tank pressure approx 3000 psi.

​

I'd want to rebuild it anyway due to the IP creep. Then you can carefully inspect the piston shaft and the ambient chamber surface. I'm not sure what you mean by 'from the LP side..." but I guess you think the leak might be coming from the piston head o-ring, not the piston shaft. I don't think it really matters because with that much creep it needs some attention anyway.

You might put it on a tank with a SPG and an IP gauge on the LP inflator hose, pressurize, then turn the tank valve off and wtch what happens over an hour or so. If the IP drops gradually, then burps back up, it would indicate the leak is on the piston head. If your leak is as slow as you say, it might not register on the IP gauge at all.

 

[scubafanatic]

I'd want to rebuild it anyway due to the IP creep. Then you can carefully inspect the piston shaft and the ambient chamber surface. I'm not sure what you mean by 'from the LP side..." but I guess you think the leak might be coming from the piston head o-ring, not the piston shaft. I don't think it really matters because with that much creep it needs some attention anyway.

You might put it on a tank with a SPG and an IP gauge on the LP inflator hose, pressurize, then turn the tank valve off and wtch what happens over an hour or so. If the IP drops gradually, then burps back up, it would indicate the leak is on the piston head. If your leak is as slow as you say, it might not register on the IP gauge at all.

The 1st-stage in question was fully rebuilt (not by me though), using new kits from vintagedoublehose, about 3 months ago, 0 dives since full rebuild, I was on a 'staycation' this week and had time to run 4 sets of MK5/109-156's through final tweeking in the tub. I've got several IP gauges, including the scubatools inline adjustment tool with IP gauge. The leak is somewhere towards the turret side of the stage, as I'm able to watch the bubble form/grow on that edge of the ambient chamber hole for a few seconds before it floats up, and that's the 'quadrant' or edge the bubble comes from.

The IP starts @ 125 psi, gradually (I'll have to time it tomorrow, I'd estimate 5 min from memory) creeps up to about 135 psi and locks up there, so the IP is never dropping once it locks up at 135 psi. I'm not sure what is the definition of acceptable/normal IP creep over what timeframe ? However, I had the reg fully pressurized, I did not turn the tank off and check to see what the IP does under those circumstances, I'll try that tomorrow too.

Of the 4 reg sets, only this one set is having this 'issue'.

So far, it sounds like I may have a problem with a piston head O-ring OR a piston shaft O-ring (assuming I have no physical damage to the actual piston or ambient chamber surface).

OK I'll gather a bit more data tomorrow but it's sounding like I'll have to have another full service done (maybe I'll be lucky and it's just a buggered up HP O-ring)

Thanks again,

Karl

 

[halocline]

I'm guessing that you'd have to leave the tank off and pressurized for a long time (maybe an hour, but that's a guess) and check both gauges periodically. If there's a leak, the SPG is going to drop over time. If the leak is from the ambient chamber, the IP would also theoretically drop until it allowed the piston to separate from the HP seat to separate just enough to cause a sip of HP air to get past and bump the IP back up. This is common with leaks in the balance chamber on 2nd stages. I suspect with your slow leak the gauge would not move perceptibly, it would be more like the hands on a clock.

But 10 PSI creep is way too much, I would personally not tolerate that on my own regs. I'd pull it apart and check the piston knife edge with a jeweler's loupe. I have no experience with the aftermarket HP seats, but if the piston knife edge is flawless, something's not right with that particular seat. It's the only other possibility for IP creep.

You could try breaking in the seat a bit; leave the reg pressurized overnight and bump the purge several times; that might help the piston cut a more reliable sealing groove in the HP seat. I've occasionally done this with MK10s, they have a tendency to creep more than MK5s. All of my MK5s are rock solid, zero creep with a new seat. In fact it takes years for them to start creeping. SP really got it right on those regs.

 

[scubafanatic]

Update: now I've got a second MK 5 1st-stage also leaking in the tub. 1 bubble every 3 - 5 seconds from an ambient chamber hole. 1st-stage orientation underwater doesn,t really seem to affect leak rate. Have not had time to see if IP is creeping yet on this one. Guess I'll leave both units pressurized overnite tonight and see if it makes any difference. I think I may have already had them pressurized for some hours already but I've lost track exactly so I'll just do it again tonight to confirm it's been done.

 

[redacted]

Update: now I've got a second MK 5 1st-stage also leaking in the tub. 1 bubble every 3 - 5 seconds from an ambient chamber hole. 1st-stage orientation underwater doesn,t really seem to affect leak rate. Have not had time to see if IP is creeping yet on this one. Guess I'll leave both units pressurized overnite tonight and see if it makes any difference. I think I may have already had them pressurized for some hours already but I've lost track exactly so I'll just do it again tonight to confirm it's been done.

Orientation will not effect leak rate but it should cause a delay in leak observation while an air pool fills on the leaking side. If the leak is fast, you may have to clear the ambient chamber of air, impart the desired tilt, and then turn on the gas to see the effect. It is much easier to troubleshoot when you know which side is leaking. With that faster leak, halocline's method of observing IP should also work and be easier. Or try both just to be sure.

 

[halocline]

One thing I have noticed when leak-testing regulators in the tub is that sometimes it takes a few minutes for every bit of air to escape the ambient chamber. Is it possible that the bubbles you're seeing are just that? I guess if you shake and roll the tank around a bit and then leave it for several minutes, then come back and you find a steady bubble stream, that's different. But a few times I have submerged regs in the tub and thought I saw a slow leak, when after a few minutes it stopped.

Unfortunately you might have a scratch in the HP o-ring journal, which would really suck. That means goodbye IMO, that's one area where you really don't want to tolerate any leak. I guess you could try to look in there with a magnifying glass and a strong light, maybe you can see something. Or maybe it's just a matter of really completely cleaning the area, making sure every remnant of grit, grease, and dust is gone, cleaning up the piston shaft with some micromesh, and carefully installing a new 90 duro o-ring.

But these are all wild-ass-guesses.