Weak braking power - despite new master cylinder


#1

'98 Pathfinder, 116 000 miles. Brakes are weak - can’t lock up the wheels (I mean: can’t brake hard enough to engage ABS).



When I brake, the pedal is a bit mushier than it should. If I press hard enough I can get the pedal to the floor - yet the car just does a quick slow down (instead of an energetic stop with ABS kicking in).



I did a thorough bleeding job (replaced all the brake fluid in the process) by pressure bleeding with a Motive Power Bleeder. Fantastic tool, but no change.



Replaced master cylinder and bled everything according to the Haynes manual (bench bled the MC, then bled the whole circuit again). No change.



So at this point I have no clue what could be wrong. Air trapped in the ABS circuit? (I re-bled today after a test ride, and the rear load-adjuster sputtered a few more bubbles - where did they come from? I bled almost a cup of fluid out of that valve yesterday! This is what leads me to think that some bubbles could be trapped in the ABS circuit).



I cleaned all the braking surfaces the last time I changed the pads, so there should be no oil contamination there.



I can also confirm that I’m not leaking fluid - tank stays at same level during test drives (LOTS OF BRAKING).



The fact that I can get the pedal to the floor while driving tells me the booster is working fine.



Yet I can floor the pedal, so it must be air, right? WHERE IS IT! :wink:



I must confess one thing: the first time I bled this vehicle (and this may be when the problems all began - it was a while ago), I forgot to disconnect the battery to disengage the ABS as Hayes recommends. Could that be the source of my problem?



Anyway, thought I’d ask for advice…



TIA


#2

Mushy brakes on a 10 year old vehicle with 116,000 miles can be more than just the product of air in the line (although I’m sure you have some). Brake lines that expand going into the calipers and that swell can feel mushy. Having driven an older Pathfinder like yours I recall that the brakes were always mushy and I never recall being able to get the ABS to engage on any surface but ice. Also, have you put the car on a lift and had someone inside gently depress the brakes while you hand rotate each tire? You need to check for sticking or weak calipers.


#3

Thanks for taking the time to reply. I had not thought of lines that swell - I guess it’s a possibility.

As a follow-up I did some more testing today and found that if I drove it down a hill at the right speed, I could hammer on the brakes and engage the ABS. I did this about 10 times and re-bled all the lines - and a bit of air came out of the front left wheel bleeder valve.

Braking power after that was marginally better, so I’ll be doing a bit more hard stopping/bleeding for a while until bleeding yields no more air.