'98 V70 Hard Brake Pedal, "Popping" Sensation

[bmdubya1198]

Well I guess I'll finally give my daily a little attention to take my mind off of my projects!

I've had an issue for a while, probably since I changed my brakes about a year and a half ago, where the brake pedal just gets really hard sometimes. It's not too noticeable until I realize I'm putting most of my weight on the brake pedal just to stop the car from 35. The pedal is pretty stiff as it is, which is why it's not always immediately evident.
I've noticed it much more lately, especially on some of our rougher roads when I'm trying to slow down for construction and whatnot that it takes a lot of effort to slow down or stop and it feels like the ABS is kicking in (and it definitely has a few times).

The other issue that has been very noticeable is the brake pedal giving a "popping" sensation when it stiffens up. It feels like it's hitting steps while I depress it, and the brakes are less effective.

I don't know if it's an issue with how the brakes were bled (that was a project in itself, I could never get the pedal to feel "right," it was either way too soft and it went to the floor and the brakes barely worked or it was like it is now) or it's an issue with the booster. I have a feeling that the problem could be the check valve (is there only the one on the booster for 1998 models?). And just maybe that's what is causing that hiccup that my car has had since day 1, which interestingly enough happens most often while braking, and ONLY happens while I'm on the brakes (never done it while idling in neutral or park).

I just want my brakes to feel more linear than they are now. Plus, they seem to be too weak most of the time. I know the problem has to be within the power brake system (with the exception of the master cylinder, already replaced that a while back and it's not losing fluid) rather than the brakes at the wheels. Those have been completely redone, I replaced the seals in all four calipers as well as changed the front hoses and of course rotors and pads.

Sorry for the long winded and possibly scattered post, this is just one of those weird issues that I want to get taken care of. Is there a way to test the check valve, and am I that far off with that being the potential issue?

[scot850]

Sounds like the booster may not be working.

Check the pipe to the booster from the inlet manifold has not split with age. Not all V70 P80's route the pipe the same, but it usually runs over under the battery tray and then under the airbox and up to the booster. If it has not been replaced then it makes sense to do that as they can crack with age.

2nd issue is the booster valve. It is a plastic valve that the booster pipe connects to. They can fail preventing vacuum holding in the booster. Pull one form a suitable junkyard car and swap it out and see if that makes a change.

3rd issue is how are the calipers? Rears are prone to seizing but fronts can too, especially when re-built units. Like axles re-built units are a crap shoot.

4th issue is when where the flexi-pipes last replaced? They can fail internally preventing all brakes working, and making it harder to bleed the system. A pressure system for bleeding the brakes like a Mity-vac (?) or similar really helps on these cars.

Neil.

[bmdubya1198]

Sounds like the booster may not be working.

Check the pipe to the booster from the inlet manifold has not split with age. Not all V70 P80's route the pipe the same, but it usually runs over under the battery tray and then under the airbox and up to the booster. If it has not been replaced then it makes sense to do that as they can crack with age.

2nd issue is the booster valve. It is a plastic valve that the booster pipe connects to. They can fail preventing vacuum holding in the booster. Pull one form a suitable junkyard car and swap it out and see if that makes a change.

3rd issue is how are the calipers? Rears are prone to seizing but fronts can too, especially when re-built units. Like axles re-built units are a crap shoot.

4th issue is when where the flexi-pipes last replaced? They can fail internally preventing all brakes working, and making it harder to bleed the system. A pressure system for bleeding the brakes like a Mity-vac (?) or similar really helps on these cars.

Neil.

Thanks, I might go ahead and change out the booster vacuum hose anyway, as well as the check valve.

I redid the calipers myself, and the rears were pretty seized up. The front were not bad. I hear ya on the reman axles being a crap shoot... my Napa one has caused my transmission to leak from the output shaft since I put it in!

I changed the front flex hoses, the rears are original. They looked to be in good condition though, so that's part of the reason I haven't bothered.

[scot850]

As long as you get a good flow of fluid out the rear calipers when bleeding them then chances are the rear flexi's are probably ok.

Just make sure all the calipers are free, but I think the issue may be with the booster system.

Neil.

[FLXC90]

W; make your testing easier, get a piece of tubing you can run on top of the motor temporarily, then work on routing one the right way after you've done your diagnosis.

[bmdubya1198]

Same here, and I'm hoping a new check valve and hose will fix it.
Fluid flow at all four calipers was just fine. I used the Harbor Freight vacuum hand pump to bleed them.

[bmdubya1198]

W; make your testing easier, get a piece of tubing you can run on top of the motor temporarily, then work on routing one the right way after you've done your diagnosis.

Good idea, that's probably exactly what I'll do!