Inconsistent Brakes

[87GrandMarq]

I’ve noticed recently on my 90 MGM that sometimes when I brake the pedal will go to its usual spot, then it’ll start to lose braking power so I have to keep pushing it further if that makes sense. Then sometimes I don’t have to give it much pedal to stop. It kind of just happened out of the blue. Also after I turned it off today I hit the brakes to see how many times it would take to make the pedal hard and it took one time. Is that indicative of a bad check valve? Any ideas? Thanks.

[Brown_Muscle]

I would say the brake booster is going bad

[packman]

That would be my guess; or you have a bad fitting or a line that's going bad. The CAI in my MGM laid up against the brake line going to the ABS module. That killed the line at the fitting and I had the same problem you have.

[sly]

Possible booster leak, booster check valve going bad, possible hydraulic leak inside the master causing it to bleed down (had this issue with one vehicle).

[friskyfrankie]

I'd change the check valve first. More than likely, not the cause but the easiest and cheapest fix to try first. Move on from there....

[sly]

Check the hose to the intake manifold as well. If it's collapsing or turning to dust, that could also be causing the problem.

[87GrandMarq]

I’ll take a look maybe Sunday after work if it’s dry. I have a lot to go on now, thanks. How do I check the booster?

[Kodachrome Wolf]

I’ll take a look maybe Sunday after work if it’s dry. I have a lot to go on now, thanks. How do I check the booster?

A vacuum pump and gauge tee'd in could be handy to see if its leaking internally.

FWIW, if for whatever reason it does happen to be the booster, I'll make a suggestion to use the '92-'00 dual diaphragm unit (non-ABS if mentioned). It does move the master cylinder out a little further, but of the vehicles I've driven with them fitted, the pedal feel has been better than the '79-'91 single diaphragm unit.

[GM_Guy]

Any idea what the difference between the civilian and police booster is? Civilian 1992-2000 Police 1995-2000
(The 92-1995 make an abs/non-abs callout, non-abs is the same boosters as shown in the attachment for the 96 & 98 years)

[Kodachrome Wolf]

Any idea what the difference between the civilian and police booster is? Civilian 1992-2000 Police 1995-2000
(The 92-1995 make an abs/non-abs callout, non-abs is the same boosters as shown in the attachment for the 96 & 98 years)

The police booster is supposed to be 223mm in diameter as opposed to the 221mm diameter the retails one were, but now it's showing both are the same size on RockAuto. That doesn't make any sense because there would be no reason to differentiate the two.

I know the '92-'95 ABS boosters use metric studs with a different spacing between the two compared to the '92-'95 non-ABS and '96-'00 boosters which share the same stud thread and width as the '79-'91 boosters.

[gadget73]

My understanding was also that it had to do with the stud spacing, but hadn't heard of a size difference. Can't imagine an extra 2mm would be enough to make any real difference. I honestly was not aware there was a different part used on police cars.

I thought after the mid 90s the master cylinder was also the same ABS vs non-ABS. I know the early 90s ones were different.

[GM_Guy]

Interesting. Always something new to learn with these panthers.

[87GrandMarq]

I finally got some time to mess with it. I swapped out the check valve and they still act the same. Sometimes when coming to a stop I have a good pedal. Sometimes it’ll sink to the floor. I’m going to replace the master cylinder anyway as it is original. I hate throwing money at a problem so I might mess with it more tomorrow when I have some daylight. I’ve been trying to diagnose an issue with the ac so another thread will be made on that.

[sly]

If you're in for the master, do the booster as well. Then you will only have to open the brake lines once.

[87GrandMarq]

That’s probably what I’m going to end up doing. I’m convinced it’s the master. Getting in it again last night and today the brake pedal is almost always spongy and will sink with normal pressure.