master cylinder question

[Dauber]

I have a question on my master cylinder. I have a 3/4 ton 83 burb. It has a cast iron mater cylinder and the hydro boost behind it.
My question is when I remove the cap to the MC the larger reservoir is full of brake fluid an the small reservoir is always low and has been completely empty at times. I can not find a leak anywhere around the truck.
The rubber seal inside the cap says "fluid side" above the large reservoir and nothing above the small one.
My experience with MC's is that the large one is for the front brakes and the smaller one is for the back breaks. This MS looks like it draws fluid from both reservoirs and then out to the break lines equally, but this is probably not the case I just can't see where the lines were going inside that cast iron MC. The lines coming out of the MC are one in the front (pointing towards the front grill) and one line coming out the middle of the MC. Any help explaining this thing would be appreciated.

[DmaxMaverick]

As your brake linings wear, the fluid level will go down. If the rear is emptying, you are leaking it somewhere, the linings are wearing at a faster rate, or the MC is worn out and the cylinders are bypassing (fluid passing from the rear to the front via the bore). The "fluid side" label just helps to keep you from installing the gasket upside down. The reservoirs don't (or shouldn't) share fluid unless one overflows into the other, if it has a notch or bore between them (some do, most don't).

You could be losing fluid in the rear at the slaves, but at a slow enough rate it isn't getting out of the drum/backplate area. You'll have to pull the drums and confirm they aren't leaking. The rear reservoir isn't very big, and won't leak much before it's empty. Also, a ruptured line may have such a small hole it squirts away from the truck, but only when it's applied.

Ultimately, it could be normal for your brake wear trend. More wear means more fluid loss at the MC. If you don't have a leak, you'll get it all back (or most of it) when you compress the pistons during a service.

[murphyslaw]

I just replaced a section of line on my buddies ford 6.9 that was doing that we would top it off run across town then he would b*tch about not having any brakes so we were sittin in his driveway the other day and i jumped in and with out starting it pressed the brakes(after refilling and priming the mc) and I hear this sssssssssss i was like WTF is that? so told him to look under the truck did it again sssssssssss and he saw it turns out that the lines run on the top of him cross member and the one was rotten enought to be leaking under pressure but squirting just right to not be getting anything wet. took it to my shop and cut the line out and put in a new piece

[Dauber]

Thanks for the replies guys. I am thinking this is either in the rear brakes or a small leak in the line. I am going to pressure wash under the engine tommorow to clean up the power stearing leak from the pressue line I just replaced, maybe I will find something after that or I will pull the drums off this weekend to take a look.