What next to check to fix my PM?

[TurboSlacker]

I'm stumped and would appreciate some guidance.

My 87GN has always stopped poorly 10 years and counting. I'm determined to fix it this year. I prefer to fix the original powermaster rather than switch to vacuum for now.

No matter how well I bleed the brakes I still cannot stop quickly, lockup the tires EVER, or foot brake bump into the second stage beam with over 2psi of boost. Not very reassuring when at highway speeds or 115+mph at the track.

Here's the part changes thus far:
- replaced switch to grey one and new accumulator in year 2000
- installed brass proportioning valve
- Russell stainless brake lines on front wheels
- verified zero brake fluid leaks anywhere
- bleed brakes using MitiVac at wheels many, many times
- dual large Raybestos shoe pads on rears
- Performance Friction Carbon Metallic pads on fronts

I thought the service I performed today would finally fix it but no such luck
- bench bled accumulator and master cylinder on car per good TB instructions; certainly saw air bubbles which I successfully purged.
- bled all four wheels in proper order
- fresh DOT 3 fluid used
- reservoir fluid stays clean and without bubbles
- pumping down ACC takes 15 pumps with key off (normal); hard pedal moves 2 inches (normal)
- energizing systems key-on takes 12 seconds (not too bad)
- never have the brake light come on the dash
- test driving: the braking is consistent and predicable, just not strong

What do I diagnose or part swap from here?
Perhaps rebuild master cylinder with fresh seals?
I could find a shop to measure the hyd line pressure at the wheel although I already think it's obviously below spec the way the brakes are performing.

Greatly appreciate help

 

[TurboSlacker]

More info
Just tested the motor cycle time. The motor does not continuously run and does properly turn off in 12 seconds. BUT, it does run ever time I touch the brake and it ends with a fluttering sound and feel thru the pedal. It even dims the interior lap quickly when it does this. Perhaps the relay is bad or indication that the motor/pump assy is bad.

Whatcha think?

 

[JP87GN]

I don't know what to tell you, except you better get it fixed soon because I ordered an all Billet transmission and I'm ganna put you in you place this year! LoL!!

I do know what you mean though. My GN stops on a dime but my T-type has always had Powermaster issues. I just don't want vacuum brakes either.

BTW, How you doing? LoL!:biggrin:

 

[TurboSlacker]

Doing just fine, Josh. Puttin' the smack down, just tired of not stopping worth a crud. Never bothered with it before because it's not terrible just annoying and a slight safety concern. Of course it bothers me the most that I can't brake bump into the staged position - don't want to simply hand you a race instead of kicking your butt

Nice job collecting pretty parts...now put that GN junk together and give me your best shot !!

 

[davidf]

Any reason you guys are shy of the vacumn setup? A friend has it and loves it. I'll be switching over soon along with the S10 brakes setting in my garage.

 

[Bobs rare 6]

Hydro-Boost!

 

[salvageV6]

If the motor runs every pump it may need an accumulator.

That isn't going to fix the non-stopping part though.

How are the rotors? Glazed? Were they turned or replaced with the new pads?

That's the #1 issue with poor stopping if you have pressure in the lines.

 

[TurboSlacker]

Any reason you guys are shy of the vacumn setup? A friend has it and loves it. I'll be switching over soon along with the S10 brakes setting in my garage.

Nothing wrong with vacuum it's just a personal preferrence on how the brakes feel. The GN I tested with vacuum brakes had a pedal feel that was soft and squishy. It stopped the car just fine. I'm sure I would be fine with it if I drove it regularly.

 

[TurboSlacker]

Update on progress for those you have similar problems.

Changed the brake switch and solved the fluttering sound and feel thru pedal. Definitely resulted in improved brake application being more progressive and less pedal movement. Still unable to lock up the tires at any speed but closer.

Changing accumulator did not improve anything. Same brake application feel so left mine on. Performed more bench bleeding on the car and at wheels just to be sure all air is out.

Then I did the old drive in reverse and slam on brakes to set the rears. That helped a little more as well. Now I'm much closer!

Inspected the Raybestos rear shoes (double long) and drum, they looked good. The Carbon Friction front pads are down to 20% and the rotors are slick and near mirror reflection. I'm guessing that qualifies as "glazed." So I'll get new rotors and hi-perf pads next. I'm confident that will help a lot as well and be good enough for now.

Thanks for the tips guys. Hope this info helps someone else.

 

[TurboSlacker]

Final update . . . problem fixed!! Best brakes this GN has ever had in it's 23 years of life.

I replaced the stock (glazed) rotors with new Powerslots and new HAWK HPS pads. Also swapped out the original stock calipers with rebuild stock ones. Wow what deal - only $13 for one completely rebuilt, fresh caliper after returning cores. Bought through Schucks/O'Reillys.

Initially after installing everything the pedal went down halfway before feeling normal braking, so I knew there must still be air in the system. Bled the calipers better and that solved it. Also, the pads were not biting much yet, so once I had full brake pressure, drove it and did the old 35mph quick stop, 45 quick stop about 6 times. Holy cow, I've never had brakes heat up so much that they start smoking That was sweet! Smelled like a freakin' race car LOL.

After cool down it had the best braking EVER. Now I can actually lockup the tires if I want to and hold as much foot brake boost as the torque converter will allow - mines a 3000 stall NL.

Everything is finally great on the street. In July I'll test it on the track. I'm confident it will now allow me to bump brake properly into the staging lights.

Summary of tips for future searchers of this topic:
1) make sure the accumulator and pressure switch of the PM is functioning properly. I borrowed known good ones from other GN buddies.
2) fresh fluid and bleed the brakes at the master cyclinder - accumulator first, then bleed master cylinder where the front/rear lines connect. Then bleed at the wheels rear p/s, rear d/s, front p/s, front d/s. If you change calipers be sure to throughly bleed ALL the air out of them. You'll know it's right when the pedal moves very little (appx 1 inch) before you start to feel strong brake pressure. Should not be able to bottom pedal to the floor under full leg strength.
3) fresh rotors and pads. Follow proper initial heat cycle for proper bedding of material - this step makes a huge difference
4) replace caliper, if needed or desired, while swapping out rotor. At least do rebuilts if yours is still the original stockers.

Beyond this it could only be the power master motor, brake cyclinder itself which at this point you've got a decision to make: either replace with good rebuilt or new powermaster system (expensive) or make the swich to vacuum or hydraboost system.

A fine working powermaster is nice to finally have! But as is well documented, it does take maintenance to keep the system running properly.