PM question from GNX 258 thread.

[TURBOPOWERED68]

Originally Posted by turbojimmy

That's why I asked the question I did above. It's easy to blame the PM given its spotty reliability history, but they're not 100% sure it was the cause. I'd say odds are pretty good that it was, but you never know.

I'm perplexed by the fact that the brake lights didn't come on. Obviously he hit the brakes. Is the implication that when the PM fails you can't even move the pedal far enough to light the brake lights? I know people have had them get rock hard and lose their brakes, but when my PM failed it simply leaked brake fluid into the car.

My PM is only about a year old but now I'm worried about it. I think.

Jim

Jim
That was my next Question.
I now that with Vac Brakes if you loose Vac you can still push through the Booster and stop the car and even lock the tires (unfortunately it happen to me a few times)
And with a dead HB on my Astro I was also able to push through it to stop the van (not easy at all but no bent brake pedal here) .

I really need to now this about the PM system because if that’s the case call me paranoid my PM is gone before it goes bad.

[turbofish38]

I've had a few PM failures. I even had one at the track like John S but at least the shutdown area was long enough and I didn't wreck. The way the PM is designed you can still stop the car if it fails. The pedal will be stiff and hard but you will be able to stop the car. Now you wont be able to panic stop it and its going to take a longer distance to stop but it will still work. Usually these things just dont lock up for the heck of it. You should get plenty of warning that it is on its last legs or that a problem is coming down the road. Dont ignore the warning light. And if the motor and pump seem like its cycling a little longer than thats the sign you need to do some maintenance. Whatever you do be carefull if you panic and use the parking brake. You'll have zero control of the rear end with the rear wheels locked up, especially at high speeds and most likely you'll end up smacking something with the rear of the car instead. One of the first things they teach you at a high performance driving school is that if you do get into a situation hit it head first if you can. Thats what the car is designed to do, from a safety stand point.

[V6RACER]

I had a head gasket let go on a 9 second pass, when the engine died, no vac brakes.. I now have manual brakes on the car.

[johnplogii]

In the past when I have had powermaster problems, my light will come on & I still get 4 to 6 pumps of the pedal. If this happens just apply the brakes once, don't pump them or you will lose brakes in a hurry. I also have read that it takes 700-800 lbs of pressure on the pedal to stop the car with no power assist, and I would have to agree it feels like it.

[zeus87gn]

When my P/M finally went I was trying to pull in my driveway. I was on the pedal hard enough with both feet I lifted myself out of the seat and was hanging onto the steering wheel. I had just enough pressure on the pedal to keep the car from going into the ditch. Stopped just in time. I was only going 45mph...not 130.

I read the thread on GNX258. Saddening. I agree that its good it was doing what it was made for and glad to hear it will be rebuilt. It will be the bionic GNX as it will be better than the original.

Yes, there should have been enough to make the car stop. Question is was it enough to stop in the distance given from 130mph. Typically, there are between 6 and 9 pumps left in the pedal after it stops working for whatever reason. The brake light in the dash should come on before you run out of pressure. The pedal get's hard when you run out of pressure. I would think the brake lights should have come on unless the P/M completely jammed and did not allow the pedal to move at all.

If I may make a recommendation...
Remove the powermaster if you can (if there is anything left of it) and test it by the diagnostic procedure. It will tell you what the condition of the P/M is (was).
If it is damaged and isn't functional, pull it down and check the condition of the orings on the power piston if it isn't damaged. If the orings are blown, it's a good indication of the culprit. Also check the condition of the cylinder where the power piston moves. If is has bad scoring in a circular pattern the piston may have gotten jammed.
You can also check the acc ball on a known good P/M with the diagnostic procedure if the acc wasn't damaged. Hard pedals are usually the first sign of a dying acc but usually don't go out all at once.

I will be glad to answer any questions you may have.
Hate to hear that the P/M may get another slap in the face.
Unfortunate...


I have to ask (with the utmost respect), how well has this P/M been maintained over the years?
Is it still original to the car?

[turbojimmy]

Originally Posted by TURBOPOWERED68

Jim
That was my next Question.
I now that with Vac Brakes if you loose Vac you can still push through the Booster and stop the car and even lock the tires (unfortunately it happen to me a few times)
And with a dead HB on my Astro I was also able to push through it to stop the van (not easy at all but no bent brake pedal here) .

I really need to now this about the PM system because if that’s the case call me paranoid my PM is gone before it goes bad.

Thanks for breaking this out into a separate question. I was taking the GNX 258 thread off-topic.

Jim

[Quiky One]

Originally Posted by zeus87gn

Question is was it enough to stop in the distance given from 130mph. Typically, there are between 6 and 9 pumps left in the pedal after it stops working for whatever reason. The brake light in the dash should come on before you run out of pressure.

Not when the electrical motor fails..When it fails you HAVE NO BRAKES!!!!!!!!!! When the motor fails no lights come on...

[notacarlo]

I have vac brakes and the only thing negative I can say about it is that when you hit the brakes THEY APPLY. Real easy to lock'em up. Other than that, under controlled braking it stops pretty well. I like them better that the PM.

[Turbo1dr]

If the car has a line lock has anyone thought of the line lock as the cause of the very hard brake pedal? I know if you pinch off all of the brake lines the pedal won't go ANYWHERE and the brake lights will not come on it's so hard! IF the brake lights did not come on then it may have been a combination of the line lock not releasing AND when he hit the brakes the proportioning valve may have switched and caused him to have no brakes. Does that sound right or am I just confused again?

[turbojimmy]

Originally Posted by Turbo1dr

If the car has a line lock has anyone thought of the line lock as the cause of the very hard brake pedal? I know if you pinch off all of the brake lines the pedal won't go ANYWHERE and the brake lights will not come on it's so hard! IF the brake lights did not come on then it may have been a combination of the line lock not releasing AND when he hit the brakes the proportioning valve may have switched and caused him to have no brakes. Does that sound right or am I just confused again?

In the other thread they said they hadn't ruled that out, but it's not likely because the line lock was on a momentary switch. There could have been an electrical malfunction that activated the lock I guess. My pedal gets mushy when I use the lock though. The BRAKE light comes when I push on the brake pedal right after I release the lock, too.

It's definitely a perplexing situation. I'm not ready to condemn the PM yet. Sorry.

Jim

[TURBOPOWERED68]

Originally Posted by turbojimmy

Thanks for breaking this out into a separate question. I was taking the GNX 258 thread off-topic.

Jim

No problem
I didn't want to HJ the thread ether especially since it was a very scary moment for him and his family.
And it was more of well wishing, happy every body is ok type of thread.
It just didn't feel right for me to go on about brakes.

[TURBOPOWERED68]

Aren’t the line locks tied into the front brake lines?

If the line lock activated before the brake was applied wouldn’t at least some of the pressure be released into the or towards the rear brakes? I know the brake system does not tie-in the front and rear brakes just thinking of it from a pedal travel point of view.
I mean just enough to turn on the brake lights.

I checked 2 of my vehicles yesterday and with the engine off (not running and no key in the ignition) and I was able to turn on the brake lights by pushing Very lightly on the pedal.
1-was my 95 astro with HB Brakes
2-was my 94 Subrb with Vac.Brakes
I did not get a chance to try this on the Regal with the PM I will give it a try tomorrow.
If the car is off and no key in the switch the pedal should not move at all not even enough to turn on the brake lights?

Sorry guys for not letting this GO.
But I plan on DD my Regal and fast reliable stopping means a lot to me.
And I need to know how I will address the fast reliable stopping issue.

[zeus87gn]

Originally Posted by Quiky One

Not when the electrical motor fails..When it fails you HAVE NO BRAKES!!!!!!!!!! When the motor fails no lights come on...

Not necessarily true. If the motor fails with the system pumped up, you still have reserve presure in the acc ball. Even if the motor were to loose electrical power, you would stil have that 6 to 9 pumps reserve. IF the P/M failed, then the question would be WHEN did it fail. Why is another discussion.

[psycho6cyl]

Originally Posted by TURBOPOWERED68

And I need to know how I will address the fast reliable stopping issue.

i have been through most brake problems w the GN. The pm for me is too unreliable. I went to vac brakes and the only problem i encounter is that if you boostin and ya gotta do a panic stop you will get a hard pedal for a few. other than that a-ok.

[zeus87gn]

Originally Posted by TURBOPOWERED68

If the line lock activated before the brake was applied wouldn’t at least some of the pressure be released into the or towards the rear brakes? I know the brake system does not tie-in the front and rear brakes just thinking of it from a pedal travel point of view.
I mean just enough to turn on the brake lights.

Actually, the front and rear brakes are tied together...right in the master cylinder. You've got to understand how a master cylinder works.

The P/M (basically) has 3 pistons. In order from the firewall out - the power piston, the front wheel piston, the rear wheel piston. They all move together when the pedal is applied.

Line locks are installed between the prop valve and the P/M (single locking mechanism operating on one line), correct? If it's activated when the brakes are on, they lock the front wheels. The pressure is on the downstream side of the lock when the P/M releases and you can do your burnout. It's been done so there shouldn't be any problem there.

If the lock were engaged before the P/M applied pressure, it wouldn't allow the front wheel piston inside the cylinder to move but enough to put pressure against the lock...and there isn't much volume of fluid to be moved. The brake pedal may travel slightly but only what it would take to apply pressure to the lock. The rear wheel piston would barely move and the rear brakes would have hardly any pressure to operate.

Would the front wheel piston allow the pedal to travel enough to make the brake lights come on?
Possibly. I would want to test it but I don't have a line lock. This is just my theory. But this can also happen with vacuum brakes as those master cylinders are similiar in orientation (i.e. front wheel piston towards the firewall).


So the possibility is there...if the line lock activated.

[TURBOPOWERED68]

I got your point
Let me say I have never installed or used a line lock.

I was under the impression that they get installed AFTER the prop valve.
I just thought that it being installed after the prop valve it would or could compress/pressurize just enough fluid to turn on the lights.

But it being installed before the prop valve I could see it not having enough of a distance/volume of fluid for it to compress/pressurize to allow the pedal to move.