I’m using a blow off valve rated at 160PSI. at about 130psi, it starts to hiss a bit. never went above that to see when it pops since I only set my pressure switch to 120psi max for longest compressor life.
when it fills up to above 125psi, it slowly leaks a little (no audible hissing) until it drops to about 118psi more or less then stops there. is the spring in my blow off valve weakening or is that normal?
it’s normal for the initial pressure to drop as long as it doesn’t continue to drop. the pressure drops as the air cools inside the tank.
the safety valve should not let air out until it reaches the designated pressure and then it’s a sudden burst. it may have a problem like debris holding it open.
thanks for hte reply, the safety valve is on the top of the tank. I usually pull the ring of the safety valve to bleed off some air before draining the tank. is that enough to get rid of the debris stuck in the valve or does that cause debris to get stuck in the valve in the first place?
if your pulling the safety valve that hasprob. loosened the spring which is why its leaking…
id replace the safety valve and drain debris out of the tank via the drain on bottom…
Pulling the safety valve could cause or fix a debris problem depending on the debris. Normally the tank is only going to have fluid in it which shouldn’t cause a leak but if you used teflon tape then a piece of that could do it.
The safety valves I’ve seen have a ring for pulling them open but I guess it’s possible for the spring to weaken if pulled too much.
the blow off valve I got for free from a huge (80gal?) surplus compressor tank. must have been the age.
I only pull it just enough for it to release pressure. I only use fingers and not pull with too much force.
might go around to getting a new one then. thanks guys. 
i always say start with the cheapest things first… if all else fails by a new tank, lol
I just got a new tank. and it fits my ride perfectly it looks stock. 
the safety valve still holds so it can wait. I just can’t go above 120psi coz it starts to leak. when it starts letting off air below that then I’ll probably start looking for one. 
Safety valves will start to bubble (leak slightly) under their rated pressure. Sudden an abrupt blow off isn’t good for air tanks. Replace yours if the pressure is significantly lower than its rated pressure when it begins to bubble.
Make sure that your safety valve is ASME certified.
Its your only defense against tank failure.
didn’t know that. how is that not good?
well, my first air system didn’t have a blow off valve so I thought an old one is better than none at all. 
gonna look around for a good quality replacement when I get the chance.