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Buck
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Discussion Starter #1 (Edited)
Very strange brake problem happened last evening. The setup is a power steering pump supplying fluid for power steering and hydraulic brake booster. Then what appears to be an AC Delco Master Cylinder dual unit sending pressure to the front (89 Mustang GT calipers) and rear wheel cylinders (94 Mustang GT calipers), rear port to the front brakes and front port to the rear brakes. A few weeks ago a neighbor of mine (very good mechanic) installed new braided lines on all four corners and replumbed some hard line on the rear axle, and did a through bleeding. So far so good, everything worked and no problems or leaks. Everything seemed fine for a few short test runs and noticed the brake peddle was lower then I liked so I adjusted the peddle linkage to bring the peddle up. Then 2 days ago the brakes started to lockup and and did want to free up, but I nursed it home. The brakes were hot and starting to smoke on all four corners. Thinking I brought the linkage up too far causing the brakes to drag getting them hot. Yesterday I adjusted the linkage again and went for a test ride and after about 5 miles and stopping a few times and checking the wheels and calipers they were warm but cool enough to touch, yippie problem solved, ready to ride. However as soon as I turned around the brakes started to lockup again and smoke bad and no way could I make it home,, so I stopped to let them cool down and hopefully free up. After about 30 minutes the brakes freed up so I started up and the peddle went to the floor.........NO BRAKES at all. Fortunately I'm on a small country road no big hills, so I put the AOD trans in low and creeped it home safely. I looked everything over this morning and nothing loose or unusual and no leeks so I'm thinking it has to be the master cylinder or possibly the hydraulic booster and the power steering still works so I doubt the pump is bad. Does anyone have any insight on what is going on here.
Thanks for any help
Buck

I forgot to mention that the hydraulic booster and Master cylinder were new last year and only about 1500 miles.
 

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Focus on the brake part of the system, not the pump or booster. Even if either the pump or booster would fail, (1) the pedal wouldn't go to the floor, (2) you will still have brakes, just not power assisted. Standard troubleshooting would be to go back and look at everything that was changed since the last time it worked.
 

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Buck
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Discussion Starter #3
edwardb, thanks for the reply. The last thing changed before failure was the peddle adjustment.
 

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Not a waxer
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Sounds like when you adjusted the pushrod it kept pressure on the brakes which resulted in boiling the fluid. When boiled brake fluid cools you wind up with a great deal of air in the system resulting in a low or non-existent pedal. I'd readjust the pushrod to assure that it has a small amount of slack then flush with fresh fluid and bleed.

Jeff
 

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Sounds like when you adjusted the pushrod it kept pressure on the brakes which resulted in boiling the fluid. When boiled brake fluid cools you wind up with a great deal of air in the system resulting in a low or non-existent pedal. I'd readjust the pushrod to assure that it has a small amount of slack then flush with fresh fluid and bleed.

Jeff
And watch for leaks at the calipers. You may have cooked the seals.
 

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Junior Charter Member
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3,114 Posts
Times two on adjusting the pushrod correctly
 

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Cross Menber
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I agree, make your adjustments first.


If you still have problems, don't overlook the possibility of a hose going bad and
not let fluid return to the master cylinder.

I put on 2 brand new hoses and within a week had a caliper lock up.
After releasing fluid via bleeder, I was able to limp the last couple miles home.
The hose restricted the flow enough to keep it from releasing.
 

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Buck
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Discussion Starter #8
Thanks All for the replies. The flex lines on the corners and the center rear were just replaced with SS braided teflon lined. no kinks, nothing pinched, they all fit very nicely. I can see how a kinked line could do this but on all 4 corners? I hate to think if they were still the rubber ones. After adjusting the linkage I was still able to push the car by hand with no restriction even with the engine running and in neutral. I'll back out the linkage to where it was and flush and bleed then keep a very close eye on it. I'm still thinking there might be something wrong with the master cylinder. I'm still confused by going 5 miles with hills and sharp curves and the wheels and calipers were touchable and within 100 feet of checking them and turning around they start acting up.

Buck
 

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Had the exact problem with a GMC truck with hydra boost ...brakes would get hot, all four, and lock up. Turned out to be insufficient clearance between the master and the boost unit rod. What solved the problem was adding shims between the master and the booster...temporary fix for diagnostic purposes. If your problem continues try loosening the master from the booster to see if there is any effect.

Also could be master cylinder compensating port not allowing fluid to return and thus maintaining pressure. Had that issue with a Dodge Colt. Brakes would lock up solid, even with the master removed from the booster, all lines attached, and "hanging in mid air" by the lines. Brakes were still locked solid with out any input to the master. A new master fixed the issue. Good luck.
 

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Check the master cylinder piston retaining circlip. If the pedal goes all the way to the floor as for example during brake bleeding, the pushrod travels through a slight arc and can contact the ears on the circlip which can bend the circlip in toward the piston. The bent ear can cause the piston to not retract all the way which prevents communication between the reservoir and the pressurized fluid - thereby maintaining some pressure even though the pedal is not applied.
 

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Sounds like when you adjusted the pushrod it kept pressure on the brakes which resulted in boiling the fluid. When boiled brake fluid cools you wind up with a great deal of air in the system resulting in a low or non-existent pedal. I'd readjust the pushrod to assure that it has a small amount of slack then flush with fresh fluid and bleed.

Jeff
THIS.

And could the fluid used have been bad? I would drain the system and start over with fresh fluid from a different source.
 

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I think if there's constant pressure due to the new depth setting it wont relax enough to let fresh fluid back in no?
 

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Crash has a point on the fluid. I have seen mixed fluid (esp DOT 5 with 3 or 4) make a man crazy chasing brake problems. Any chance when you did the teflon hoses you refilled and bled with a different type? Flush, flush again, and replace. Wilwood has extreme temp DOT 4 - good product if you expect to heat the brakes once you fix the problem.

Re: the accumulator on your hydraboost. Not saying this is part of the problem but if there is any chance the mc rod got pushed while the ps/hydraulic system was unpressurized you dumped the accumulator. You can read up on what is required to reset an accumulator dump. It can cause a host of mc braking issues until reset.

Not a root cause in this case but for awareness bc hydraboost systems are extremely sensitive to supply AND return pressure, the pump pressure you're running and the return lines from the hyrdraboost and steering rack back to the pump dedicated (not tee'd) are important. They are great systems once fully worked out, but they make you work for it. I ended buying several instrument quality pressure gauges (Grainger) and making up AN fitting manifolds that I could hook up both supply and return to check system operation. On musclecar retrofits usually ended up modifying the pump housing on Saginaws and the like with another AN return port by brazing a fitting to the housing.
 

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Buck
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288 Posts
Discussion Starter #14
I finally found the paperwork on the power brake system. The master cylinder and hydraulic assist unit were purchased in 2016 from Hydraboost and has worked fine since installation. At that time the brake fluid was from a fresh can of dot3 and used another fresh can of dot3 after the brake line work a few weeks ago. As far as the PS pump PS rack and brake assist unit, the only place that didn't get emptied was the rack which may have some unknown brand of fluid in it, but after installation of the hydraboost it was filled with a good PS fluid, turned the wheels stop to stop several times then topped off. Again everything worked and no leaks. I'm going to inspect the entire brake system tomorrow and hopefully get to the bottom of this.
 

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Buck
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288 Posts
Discussion Starter #15
Good News.........it turned out to be the Master Cylinder. I installed a new MC, flushed the lines, bled, and checked all other fluids, and the issue seems to have gone away. I ran several short test runs applying brakes at various speeds and everything works as it should and the peddle is where I like it. Thanks for the help and suggestions.
Buck
 

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Buck
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Discussion Starter #17
Mark

Nothing wrong visually on the outside. I worked the piston and got about 1 3/8" total travel stop to stop so I think that was ok. It felt just a tiny bit sticky pushing in but smooth pushing back or returning. I pulled the circle clip and piston out and found nothing wrong inside. I'm thinking that maybe a small piece of debris may messed with the piston or clogged a hole in the master cylinder OR bad adjustment at the peddle on my part were the problem. Anyway, a new MC and some careful attention to peddle adjustment, the problem seems to be resolved.
 
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