front brakes/wheels locked up

[ltdcrwnvic]

So i was driving...luckily doing about 10mph thru a parking lot, and the front brakes locked up on me.

I parked it, assumed it was the MC, so i switched it out but with no luck, they still locked up. I lifted the front end and with the car off, the front wheels spin freely. Once i turn on the car, they immediately lock up again.

So would this be a proportional valve issue, a brake booster issue, bad calipers, or what do you guys think it is?

Thanks for the help guys!!!

[BerniniCaCO3]

weird!

I've heard of hoses being a cause-- the rubber bit deteriorates from the inside out (so not obvious looking at the outside), and creates a one-way check valve like in your arteries/heart.
But that would simply be whenever you apply the brakes. Should be uninfluenced by whether or not the car is on!

Anything sticky, be it caliper, MC, brake hose, proportioning valve (not sure how these usually fail), would logically merely HOLD pressure after you have already applied it-- any of the above would fail to release pressure once-applied.
What's so odd is that you're saying it'll be free, turn on the car (without hitting the brakes), and now they lock up. Unless, are you hitting the brake pedal to start the car?

No ABS, right? not sure if this is a lincoln and maybe they were ahead of their time already... doubt it.
or maybe you added ABS. I think some members were considering it, or have done it already-- it was a lot of work to do iirc.

Only thing having the engine on does, is give vacuum to the vacuum booster. While this makes pressing the brakes easier, I cannot imagine that your vacuum is so strong that it could auto-apply them (just as a thought experiment though, if you could draw enough vacuum, could you apply the brakes without hitting the pedal? It might be something crazy that no engine could manage however).


The fact that both fronts lock up together and that this just started, makes calipers/hoses unlikely. While they age together, failing abruptly and simultaneously would be rare coincidence.
MC would have been my guess --our master cylinders ARE split front:rear, I think; if they were LF/RR and RF/LR then it wouldn't make sense of course.
The only other thing that would influence both fronts at once would be the proportioning valve then...

I always mix them up. There are two valves... just checked. It's the METERING valve you're talking about actually. metering is for front disc/ rear drum, to start sending pressure to the drums before the discs, because the drums take longer to actuate (more travel than the .006" that you find in front disks). that way, hopefully the front and rear brake at the same time. Otherwise, the fronts brake much too soon, and lock up and skid.
Proportioning is an additional valve that lets in even more pressure to the drums the lower the car sits (because lower implies you have more weight in back-- and if the load has been moved rearwards, you need more braking power in back). Used on pickups especially. Also look for it on nicer air suspension systems I think.

[jaywish]

what was the last thing you did to the brakes and when?

[miamibob]

Calipers locking up????

[ltdcrwnvic]

morning guys....

@berninicac03: sorry about not giving you my car's specs....its an 89 ltd cv....no ABS, base model

@jaywish: the last thing i did to the brakes was change the front brake pads about 3/4 months ago.

i'd also like to add that yes, the front brakes do lock up by themselves once i turn on the car.....no i do not step on the brake pedal when turning it on. once the brakes are self applied, the pedal seems to be all loose and flimsy, as if it were just hanging there...but when the car is turned off, the pedal comes all the way back up and gets a bit stiff as if nothing ever happened.

[86VickyLX]

Your brake booster is bad and leaking internally. The vacuum of the engine is pulling the brake pedal down and applying the brakes by itself. Disconnect the booster vacuum line while its running and plug the vacuum line off and see if you still have locked up brakes. Obviously you won't have assist, but it would confirm my suspicions.

[miamibob]

Your brake booster is bad and leaking internally. The vacuum of the engine is pulling the brake pedal down and applying the brakes by itself. Disconnect the booster vacuum line while its running and plug the vacuum line off and see if you still have locked up brakes. Obviously you won't have assist, but it would confirm my suspicions.

EXCELLENT idea!!

[gadget73]

Usually if the booster is bad, its because the MC failed. Was the old MC showing any signs of leakage when you took it off?