I have an '86 GMC Jimmy with a 10-bolt rearend. About three months ago I replaced the rear brake shoes. While I had the brakes apart, I went ahead and replaced all the hardware and both wheel cylinders because one was leaking slightly. Heres where it gets weird. After putting everything back together and bleeding the brakes, the brake pedal went farther to the floor than normal and was mushy. I went ahead and replaced the master cylinder since it was old also, bled the master cylinder before installation, rebled all four wheels, adjusted rear brakes, and still had low/mushy pedal. I ran about three of the big bottles of brake fluid through the system bleeding it but it did not get any better. One thing I did notice was the the left rear bleeder would always spit out a little bit of air. I was busy for the last three months and did not have time to work on the truck. Over the past week I started looking at it again and have not been able to figure it out. Over the three months that I drove the vehicle daily, no brake fluid had leaked out. I have inspected all lines without finding any leaks and made sure all fitting were tight. Replaced all rubber brake hoses, the hard line to the the rear left, tried another new master cylinder, and replaced the left rear wheel cylinder four times and the problem is still there. I can suck fluid through the left rear with a pump, close the bleeder, the brakes will be firm at first but after pumping the pedal four or five times it gets mushy again. bleed all wheels and only get air out of the left rear. I took the truck to a local respectable garage who power bled and manualy bled the system with no luck. They also inspected the whole system like I had and could not find a leak. I have run out of options to troubleshoot this problem. I don't understand how air could leak in without fluid leaking out but it seems like that is what is happening. If anyone has any ideas, they would be greatly appreciated. Thanks for any help or suggestions.