The SSP250 has a completely different valve arrangement to the 2250. The valve uses a pierce when tightened system that has an end seal and piercing pin under a threaded retaining collar to seal the CO2 cartridge. If yours leaks when cap is tightened often you can use a large screw driver to reach into the tube and give the retaining nut a little tighter tension on the seal to puff it up a bit. It uses the 38-A027 piercing pin and 38-128 seal. These should be changed out together for best results. Repalcement is is going to require a tear down as fitting the parts from the front of the assembled gun is a fruitless exercise.
Most people overtighten caps and as a result the cartridge seal gets overcompressed over time. In the case of the SSP250 you can often give the seal a second chance by turning the retaining nut clockwise 1/4 to 1/2 turn. If it isn't piercing with normal cap tension after you do this you went too far and need to back it off a 1/4 or 1/8 turn. It needs to be situated so the seal makes contact with the CO2 prior to the cartridge getting pierced by the piercing pin. Excessive force should not be required to get the gun to pierce.
Due to the SSP250's breech layout there is CO2 pressure losses fore and aft of the loading gate and these guns can increase power considerably when fitted with the breech and barrel from a 2250. The SSP breech screw normally needs to have the head recut to fit the XT breech but it isn't that difficult if your handy.
The valve to breech seal is the same part as a 2250 but it doesn't use a sleeve like the 2250 does and there is no flat on the valve body. Detail work on the valve is required to do a conversion and the 2250 front barrel support needs to be fitted. The gun will need to be cocked separately from loading as the system doesn't incorporate a hammer like the 2250.
While your at it you want to fit the hammer sear and trigger blade from the 2250 and that will improve the SSP250 dodgy trigger considerably.
The ssp250 valve is probably one of the most efficient valves Crosman ever did as they were attempting to extract decent shot count from a gun with breech looses due to the loading gate arrangement. I'd suggest you resist the temptation to change anything in the valve before you try it because the stock unit is pretty much optimized for power and shot count.
One of the advantages of the SSP250 valve is it has a microscreen behind the piercing pin to keep any debris out of the system so it is like a MKI in that it will rarely get any foreign matter into the valve. The other advantage is a 1/8" valve stem which flows more freely and has a nearly bulletproof exhaust valve seal.
I used this valve stem (shortened) in the venerable LD pistol till we ran Crosman out of them a few hundred LD's back. After that we started making our own exhaust valves so the durability and longeivity the LD's are noted for was retained.
"NO GUNS WOULD BE A RIOT"