940 brake pedal sometimes high, sometimes low

so my 93 940, been driving it for months and haven't touched the brake system other than general inspection. the other day i was on the highway, and when i got off on my exit the brake pedal went 4-5 inches lower than usual, after my heart stopped the brakes started to grab and they worked fine other than the low pedal. go down a few stoplights and sometimes the pedal would be high, sometimes it would drop to the low position. i checked the fluid - full but old, and the vacuum line looks fine but old, didn't see any leaks or obvious brake line issues under the car.

i am going to do the quick booster check i've found in searching, but this is a new problem for me, no idea what it could be. and i really don't like a low pedal, don't want to mess up the master cylinder if it's not broken


*update
ok, bleed the brakes a little bit(old school pump pedal and open bleeder way), found some air in the right rear line, all the others looked good, just did the bleeders on the calipers. pedal is still soft, but so far it hasn't dropped(will not know till i drive it more really)

priced a new master cyl at napa, $100 but it'll take a day to get it from another store. now i'm looking on ebay and some of the new ones listed look really odd. should i get a certain brand or should it be ok to go for the one from napa/etc?

update 2
master cyl at napa wasn't even close to the same, just bought some new fluid, pulled off the reservoir and emptied it, filled it with new fluid, slowly mounted it on the master cyl to keep air out, then gravity bleed from the furthest away to the closest till new fluid came out, took forever but pedal is consistent and doesn't drop anymore. still spongy before it engages but that might just be an abs 940 thing

Just do the quick checks and report back. But the booster doesn't cause the pedal to drop.

Pump up brakes with car off, then hold pedal, see if it drops with your foot on it or if it stays hard.

A bad wheel bearing can cause the rotor to wobble and push the pistons back in the caliper. That will cause extra pedal travel.

turbotankshane said:

Just do the quick checks and report back. But the booster doesn't cause the pedal to drop.

Pump up brakes with car off, then hold pedal, see if it drops with your foot on it or if it stays hard.

Click to expand...

ok, pumped the brake with the car off, pedal came up and got hard(after the top 1in of spongyness), held for 10 seconds without dropping. turned on the car, pedal held hard, pumped it and when i released the pedal the idle dropped(that might be normal). sprayed starting fluid on the brake vacuum line and plastic booster thing, no idle change

i know the fluid is old, needs to at least be blead, the pads are not new, rotors are really old, perhaps it's just the accumulation of everything needing to be changed? was planning on new rotors/pads/fluid/etc before winter hit, but would like to figure out if this is the booster/mc or not before i go changing parts

Sounds like your master cylinder is going bad. I had a similar issue a year or so ago and a new mc fixed it

Do a visual to make sure no brake lines are chaffed/split/leaking. Especially the rubber flex lines. If all looks AOK...then consider replacing the M/C with a *NEW* unit and purge/completely bleed out all the old fluid while replacing with new fluid. (bleed the brakes for an hour and use @ 2 qts of fluid)

Don't forget to *bench bleed* the new M/C before installing it on the car!

Potentially a failing master cylinder.

That being said when mine failed it would gradually lose pressure until I reapplied the pedal, then it would work again. Only really noticed it when in traffic downhill and I was on/off the brakes a bit.

ok, bleed the brakes a little bit(old school pump pedal and open bleeder way), found some air in the right rear line, all the others looked good, just did the bleeders on the calipers. pedal is still soft, but so far it hasn't dropped(will not know till i drive it more really)

priced a new master cyl at napa, $100 but it'll take a day to get it from another store. now i'm looking on ebay and some of the new ones listed look really odd. should i get a certain brand or should it be ok to go for the one from napa/etc?