...the piston length... It has to suck air in and is very restricked in stock form. One modification some have done is to drill a weep hole in the tube to allow air in more quickly - works like a charm. Get it in the right place! You will notice that the tube has a stamping to allow air in. The newly placed hole needs to go just before the end of the stamping - towards the butt end of the rifle and then deburred with a rat tail file or the similar.
On one of the many I have owned, I completely did away with the old style pump cup. Instead, I had a 2100/2200 metal piston assembly cut in half (too long in stock form) then drilled and tapped, then cut threads in the piston rod. This made it compatible with modern, current Crosman pump cups. You can also have a flat-top piston made too that uses an o-ring. Cheaper replacement of the seal going that route; just an o-ring. The valve is a flat top valve in the gun, but I have not noticed any gains in power using a flat-top piston versus the Crosman cup.
Your rifle will produce a little over the UK limit with these simple attentions to detail.
Also, do the transfer port mod on your rifle. Do a search here, along the lines of, "140 transfer port mod", "1400 transfer port mod", etc.
"Well, I thought it was a rabbit but it turned out to be Bear Grylls in a rabbit hide."