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Discussion Starter #21
I just got the sinking pedal yesterday, talked to my stepdad who used to be a mechanic, then read through everything on here.

I found the front reservoir to be out if fluid today, and rear reservoir was definitely low. Just pouring in more fluid, I was able to get more resistance in the pedal with the engine on, but also with reduced braking. At idle, I had to jam the pedal down hard to keep it from moving, and the pedal would still sink to the floor. With the engine off, I can pump the pedal, and build some pressure, but it isn't going hard like I would expect. I didn't see any fluid on the ground, or wheels. I did open the reservoir again, and the fluid looks brown in there now. My suspicion is the master cylinder has a bad seal somewhere, but it isn't at the booster, because I pulled it off the booster and had no fluid there. Brake pedal still sinks to the floor, and I have a new master cylinder that I ordered tonight before reading through all this.
I believe your issue is hopefully less complicated than mine. You will have air in the system as a result of your master cylinder running out of fluid. You will need to fully bled the system when you replace your master cylinder. This will do two things for you: Remove the air in the system and flush out all the old brown fluid. When you remove that old master cylinder check to see if fluid was exiting it at the rear seal and going into your power brake booster. If it is dry there, then you have a leak somewhere else you will need to find.
 

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Mine does the same thing. I'm going to install a hydroboost system.
 

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I started looking into my truck and it seemed to have a lot of crap built up inside the rear wheels. When I talked to PO, he told me he recently had rear brakes redone before he sold the truck to me. That was after I ordered a new brake kit for the rears. I got to replacing the brakes and cylinders a little bit ago. Just bench bled the new master cylinder, and a friend is on the way to help bleed the whole system to flush old fluid. For a little bit, I lost rear brakes entirely. I think a large part of my issue may have been the master cylinder reservoir not having a proper seal for the lid.

I'll have a better answer in an hour or so if I'm good or not.

I also started reading up on DOT 3, 4, 5, and 5.1 fluid. Don't use DOT 5. It is silicone based, not good with ABS, and very much not compatible with all the others. The quick answer on the others (3, 4, 5.1) is that they are all compatible, but different high temperature limits.
 

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Got the new master cylinder installed, and a friend came over to help me bleed the system. I made sure to push the old, brown fluid out of the lines with the fresh fluid. A few beers later, all the old fluid was gone, and new fluid in. Engine off, the pedal pumped hard. Engine on in park, the pedal would pump firm, but seemed to have pedal fade. On a short test drive, the pedal seemed to hold just fine for me, and the brakes felt much more solid.
I think I'm good for awhile now. How everybody else is too.
 
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