If it's any consolation bruno your not on your own.
I have read many posts like yours on various forums.
When the 1377 came out it was mainly made of steel, even the piston was steel for about the first six months.
As you look at the three different phases of production I would say the Phase one pistols were the better of the three. I think they were made up to 1981, the first 3 or 4 numbers of the serial number give the date of birth (month year).
The latter phase three guns can still be made into a good pistol but the cost soon starts to stack up.
1377 £89
Crosman custom steel breach £30
Crosman .22" barrel £20
Crosman LPA MIM rearsight £30 (the original wont fit)
That's £169 just to get the top end sorted
Then as Ged says you'll need a new exhaust valve, and while you're inside the gun you may as well machine off the front of the valve chamber and fit a flat top adjustable piston to reduce the number of pump strokes needed.
That's probably the beauty of the 1377 and 1322 there is no end to what can be done to them, as below.
http://airgunartisans.com/Crosman_13xx.htm
All the best Mick