Ok, I have had this truck little over a year now. When I bought it, the guy had
just put a new master cylinder on it. Never had a problem until now, and I was pulling a horse trailer with horses, so not a good time to have brake problems.

Problem is sometimes brakes seem fairly good until I get a load or am idling.
Push on the brakes and it feels like Im pushing against a brick wall but brakes arent
stopping me, seems more frequent at low rpms or idling coming up to a stop.
Also has new brakes all around and two new rotors on front. No leaks anywhere under
neath, but there is some fluid coming out of the lid on the master cylinder. I always
thought there was no pressure on that lid. Only thing hasnt been replaced is the booster.
Could that be the problem?

 

verify your getting good vacuum and have no vacuum leaks.

 

x2 vacuum problem

 

I thought about that too. But figured it couldnt be that easy, . Makes sense though.
Thanks for the help. I will check it tomorrow. But would that cause fluid to leak around the lid of
the master cylinder? Not sure how that system works for sure..

 

On my truck the fluid had leaked and caused the shaft seal to go bad on the booster, get the master cylinder to stop leaking and replace the booster and it should be fine, make sure your rear drums are adjusted right too and your brakes should be fine

 

every master cylinder i've had on my truck has leaked around the top main seal. For sure on making sure those rear brakes are adjusted too. My adjusters have never worked right so I just adjust mine manually periodically