Brakes feel harder going uphill


#1

Folks,

I’m trying to resolve a nagging brake problem on my 98 Pathfinder. In short, braking power is weaker than it should be, but I do not have sponginess in the brake pedal. I’ve bled the whole system a few times, and even changed the master cylinder - to no avail.

While performing a few tests lately, I noticed the following behavior: if I drive up a steep hill and start pumping the brake pedal with the left foot while keeping the right foot on the gas (to keep moving uphill), the pedal gets progressively harder to press and braking action diminishes accordingly. If feels a bit like when you switch the engine off with the car still moving (in neutral) and try to brake: as the vacuum in the booster gets expended, the pedal gets harder and braking action diminishes dramatically.

I don’t think this is related to the act of going uphill; it’s just that this is the only way the engine can develop enough vacuum for the symptom to occur.

This leads me to wonder if I have a vacuum leak somewhere (either in the line or in the booster itself) that only happens at higher vacuum pressures…

Thoughts?


#2

I think you’ve got a vacuum leak somewhere. But the way you are testing uphill doesn’t work. When you’re going uphill, you have to give it gas by opening the throttle. The vacuum is generated with the throttle closed. If you’re on the gas the vacuum decreases and your brake boost goes away like you’ve noticed. Sometimes the vacuum is drawn just after the throttle plate because that’s where it can generate the most vacuum. So any throttle application pretty much kills the vacuum. I’d check the hoses for a leak too, or just replace them. You can also get a vac generating device and test if the the booster system is holding vacuum from the hose at the farthest end from the booster.


#3

Thanks rripstop - indeed, vacuum increases with the throttle closed, so what I’m seeing is normal (i.e. the reduction in brake power going uphill). I mistakenly thought vacuum was strongest at high engine power settings…

I’ll check for leaks from the hose and the booster.

There’s a part of me that also wonders if it’s possible that air may have gotten into the ABS system at some point during the last 13 years, or if the hoses are swelling under pressure - but I have no idea how to go about checking that.