Volvo: Master brake cylinder, master brake cylinder, car work

Question
Dear Jason, My girlfriend's 1989 240DL sedan is showing all the signs of a slowly failing master cylinder starting this past week. She's still driving it and while the brakes work fine almost all the time she needs to be paying attention to pump the brakes to build the pressure up when it becomes necessary every so often. I'm going to have time to fix the car in the next day or two.
We have another Volvo we now use as a parts car which is a 1990 240 DL. I replaced the master cylinder in that car 1 year ago and the brakes on that car work fine.
What I'm wondering is : considering how little wear and tear the new master cylinder in the parts car has gotten would it be an acceptable replacement (I'm assuming a master cylinder should last at least 5-10 years)or am I crazy to even consider that option with a master cylinder. I was also wondering if I do use that master cylinder from the parts car and I put a baggie under the reservoir cap to slow down leakage, I'm careful to quickly plug up the brake line connections outlets when I remove the unit from the parts car so I lose at most a few drops of brake fluid between removing it and reinstalling it to my girlfriends car is it possible I'll get lucky and not have to bleed the system? Thank you for your advice.

Regard's, Don

Answer
It's completely acceptable to take the master cylinder off the 90 and use it on your girlfriend's.  I replaced mine a few years ago, which means the original equipment lasted 23 years.  Obviously, this is one of the most important parts in the entire car, to make sure you do extremely careful work; this needs to be a 0 failure device.  I think you are fooling yourself if you think you can get away with anything less than a complete system brake bleed.  It only takes the tinest bubble in each line to render the brakes unusable, and I don't see how you would be able to replace the main cylinder without introducing some air into the system.  I would cut your losses, use the cylinder from the 90, and buy yourself a big bottle of Dot 4 brake fluid and prepare the bleed all the lines.  You'll have saved 100 for the cylinder and another 200 for labor if you do it yourself, so you'll still come out on top.  Oh, and while you have everything apart, it's a good time to check the brake pads and calipers!  Good luck!