Brakes keep locking up

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eddielopezzzz
Posts: 1
Joined: Sat Jun 07, 2025 5:05 pm

Brakes keep locking up

Post by eddielopezzzz »

Hi there,

I recently bought an 1989 Thunderbird LX and I noticed the brakes were soft. I did notice that the master cylinder was new but that the brake booster was still an older one. I thought maybe the master cylinder wasn't bench bleed before installation so I did that as well as bleed the brakes all over again. However, there was no change in the brakes. I decided to buy a new brake booster just to ensure that both the master cylinder and brake booster are new. After installing the new brake booster, I noticed that the brake pedal would get very stiff. I know this can be due to a faulty brake booster, lack of vacuum to the brake booster, or faulty check valve. The interesting thing is that the passenger front tire's brakes are clearly engaging because I can smell burnt rubber.

If there was an issue with the vacuum to the brake booster, either with the check valve or vacuum lines, the brakes would not be engaging unless I were to step on them. So I'm out of ideas trying to figure out what is causing the brakes to engage. It gets so bad that I can't even drive more than 20mph!
mcm51
Posts: 134
Joined: Sat Aug 03, 2019 7:41 pm

Re: Brakes keep locking up

Post by mcm51 »

If this Bird has anti-lock brakes, if not bled correctly it can give the brake pedal a strange feel, otherwise, if you've swapped out the master, booster, and bled out the lines, assuming this Bird does not have anti-lock brakes, I'd probably consider swapping out the calipers, more so IF they have never been swapped in all these years, I thought you mentioned that you thought there was a right front braking issue, caliper issue maybe ??......you also mentioned the booster check valve, I don't see many of those go bad, but if it did, it would create strange braking, does'nt all new boosters come with a new check valve in them out of the box ?........
If I think of something else, I'll post......but assuming no anti-locks, master, booster, calipers, perportioning valve (I hardly never see those go bad), booster check valve, and bled out correctly, that's about the whole works unless I'm forgetting something............
'95 Thunderbird LX 4.6
'86 Mustang SVO 2.3 Turbocharged
'84 Mustang GT-350 5.0 H.O.
VTCI #12973
U.S. Army 194th Armor Ft. Knox, Ky. 1977
mcm51
Posts: 134
Joined: Sat Aug 03, 2019 7:41 pm

Re: Brakes keep locking up

Post by mcm51 »

Oh, and hopefully no leaking rubber lines, that would show up as damp looking lines even if you don't ever see any drops anywhere, you will notice damp looking lines, keep an eye out for that too..................
'95 Thunderbird LX 4.6
'86 Mustang SVO 2.3 Turbocharged
'84 Mustang GT-350 5.0 H.O.
VTCI #12973
U.S. Army 194th Armor Ft. Knox, Ky. 1977
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Rusty57
Posts: 1698
Joined: Mon Jun 07, 2021 4:40 pm
Location: Iowa

Re: Brakes keep locking up

Post by Rusty57 »

In a vehicle of this age it is not unusual for the piston in the caliper to stick.

This happens when the brake fluid is not regularly flushed. When the system is finally flushed it stirs up bits of rubber from the worn seals and any rust flakes in the bore of the caliper. A piece might be caught in between the piston and the bore. This prevents the piston from retracting.
Rusty
VTCI 13079
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