doubleoh

Has anyone had a problem with not enough vacuum for brakes? Or had vacuum depleted quickly with several rapid pumps of the pedal...leading to a hard pedal and lack of braking?

Derf00

check for leaks in the brake booster vacuum line that come in from the intake manifold.

Maverick123

My mustang seems to be acting up with this also. 98 V6. Brakes have initial pressure but after about 10 seconds or so, the start to sink as if the booster had a leak. I've replaced rotors, calipers, hoses, master cylinder (original problem with leaking seals causing the brakes to clamp down), new brake booster (leaking master cylinder caused fluid to travel into the booster and eat up components), and a new vacuum hose for the booster. I'm not loosing brake fluid and i have full brake pads. ABS acting up??? Any help for our posts would be tremendously appreciated.

flash_xx

Are you sure it's vacuum and not bad fluid or air in the lines? I would replace the brake fluid with Motul RBF600 and make sure it's bled properly. Also, as said before check hose from manifold to booster.

Derf00

A lot of newer cars are using LV (low viscosity) brake fluid. I'd say check what Ford put in there first to see if it's LV fluid then confirm if Motul is appropriate. My only complaint with Motul is it's meant to be used in racing applications. That means you need to replace it once a year as it's more hygroscopic than average and only has the really good specs for a short while. The average DD gets the brake fluid replaced maybe once ever 3 years or more (if that even).

I'd lean more towards ATE Typ 200 (this one replaces the legendary ATE Blue and is good for up to 3 years) or ATE SL.6 fluid for LV.

Maverick123

Not sure if its the case with newer mustangs, turns out my air intake manifold seal above the plenum is leaking. Loss of vacuum is affecting the booster (let alone not supplying the correct amount of air to the cylinders). When you've looked at everything that has been fixed, look at whats left. So back to the posts original question, if the mechanics of the brakes are working properly, try looking at what works in conjunction with your brake system. Ex. vacuum and with newer cars, electronics.