I had my brakes installed about 2 months ago, I started to lose pressure and was told that the master cylinder had to be replaced, did that and on father's day it happend again. Does anyone know what can cause this, once it losses pressure I shift to neutral and I get pressure. A friend told me that it might be electrcal, can someone let me know. Thanks

 

Welcome to the forum. By "losing pressure" I assume you mean the pedal will go to the floor? They replaced the master cylinder, that fixed it for awhile and now it's doing it again? Have you checked all the vacuum lines? What year truck, what engine size?

 

Yes the pedal goes to the floor, it's a 2006 with a 4.7 ,8 cy. We have checked the lines and the strange is that you shift to neutral and pressure is back. I wa gone for a week and it's stopping fine, the issue is when I get into stop & go traffic, it does not do it when I'm on the highway.

 

definitely check out the vacuum lines and any connections. Had a plastic fitting crack on my T-bird once that offered up the same systems until it split completely.

 

I'll check them again, I might have missed something. I'll post it here if I find anything new.

 

They replace the booster with the master cyl or just the cyl? My booster has been making a hiss sounds for well over a year and I just haven't changed it since I always have good pressure.

 

Yeah, check to see if that booster was replaced, if not, you could think about doing that, the part is like $100+ but you may be able to do a core swap and get some $$$ back.

 

A faulty booster shouldn't make the brake pedal go freely to the floor, if anything, it should make it harder to depress the pedal with no booster but with proper hydraulic pressure in the lines. I wonder if this truck has ABS? If so, this could be where the problem resides, when the brakes were done, they may have been bled as though there was no ABS.

 

I just tried the vacuum and the pressure is normal between 20 to 22 in-hg, I also checked the lines again with soapy water to see if any leaks, No the booster was not replaced just the master cyl. but this issue was going on before it being replaced, also I haven't driven it in traffic (stop & go) which is when it does it. It does have ABS in the rear, they were not bleed, valves were locked to avoid having to do this. As Alfons said if the booster were faulty the brakes would be hard and not go to the floor. Guess I'll have to take to dealer ($$$$) to see if they find anything. Thanks B-/

 

Looking at the manual, I didn't see anything about locking the valves, but I did see a special bleeding procedure for ABS vs non-ABS.