Service Brakes - corrupted system

[BobReich]

This past December I needed to have an entire new oil tank placed on our 2007 GMC Yukon 4WD and while they were in there they also replaced a busted engine mount. Last week I needed new brakes and the garage (which I trust) told me that someone has put a petroleum based product into the system and that the seal on my master cylinder was shot and, likely, every other piece of rubber in the brake system was shot as well. They flushed the lines and replaced the master cylinder free of charge but said they couldn't stand behind the work because the system could fail at any time. Well, today was apparently that day because every dash light that could come on related to brakes is now on. When I took the Yukon to the garage (a different one) that had replaced the oil tank and engine mount back in December to ask if they would have had any reason to get into the master cylinder and/or brake system they, of course, said "no". They simply said that the guy replacing the brakes messed up by working the brake pedal too hard to get the system back up and running and that I likely had nothing to worry about. Is it even possible to replace every piece of rubber that may have been compromised if indeed someone messed up and put a petoleum based product into the brake system? Thanks!