I have a 1993 Thunderbird LX. When I have to panic stop, rare but on occasion, the brake pedal is VERY hard to press down. Almost causing me to not stop in time. Have to use both feet. All other times the brakes work perfectly. Slowly pressing them can lock all 4 wheels if I want them to. Replaced master cylinder, brakes all 4 corners, brake fluid, checked brake booster and vacumm lines, all with no effect to the problem. Scared to drive in Denver.
How about a “car person” (or two) taking a test drive in your car (with them being the driver) and see if they can duplicate the problem.
Who has been doing the brake work so far? are you competent enough to evaluate the rotor surface? I worked for one boss whose answer for all “odd” type complaints was to replace the rubber brake hoses. Will you consider using a pro?
I went to school for mechanics, 20 years ago, Wyoming Tech. (Was a tech for 2 years then needed health benefits, never went back into the field) But this just has me frustrated. My first thought was brake booster, checked out fine, next was master cylinder, replaced it with a new, not rebuilt one. Then replaced all the brakes, rotors and drums included. Also replaced calipers but not the slave cylinders out back. Lack of a close dependable machine shop makes it cheaper to just replace rotors and drums. The brakes did need replaced so was not a big deal and thought that it would eliminate another thing. My Brother-in-law and I are both stumped. He suggested I also replace the proportioning valve but I am clean out of money right now.
This sounds like your brakes are “fading” when you have to make a panic stop. Not a common problem on today’s cars with disc brakes. Perhaps the brakes on the car are simply too small (a design defect) for your situation. Are you coming down a long hill and then need to make the “panic” stop?
You might be able to improve braking performance with perforated disc (like thosed used in racing cars) and a softer pad in the disc. There simply might not be much you can do, given all the work you’ve put into the brakes thus far.
I suggest you go ahead and replace the booster and check valve. The booster is supposed to give you the extra foot power you need for those panic stops. Having to put two feet on the pedal to stop sounds exactly like the booster is not working or the vacuum that the booster uses is not available when you have to panic stop. This is too important to fiddle with.
Sounds to me like he’s having a pedal problem. Something about the panic stop is causing the pedal itself to be hard to press down. If I didn’t know better I’d say the fluid was too viscous, and sudden overpressure from the pedal is causing it to essentially freeze. I’ve seen that in other hydraulic systems, but I’ve never heard of it happening in brakes before.
this power booster is vacuum operated. normally if you apply the brake and the pedal was “hard” it would indicate the booster is bad. you also need to check vacuum pressure and the check valve. i think vacuum pressure should be around 18 lbs. you could install an in line vacuum gage and see what the reading is when you panic stop. good luck with it. i have also seen where pads or shoes that were not oe quality could cause this problem
The booster does seem to be the most likely cause. You might have someone with a strong leg slam the pedal down and hold it with the engine running while you listen for a vacuum leak at the vacuum dump valve below the pedal.
I will check the booster under major foot pressure. I checked it for leaks, and operation but never really checked it with a hard slam, duh for me really. Was sure this was where the problem had to be and was easy to check since I also thought it could be the master cylinder (binding piston) but it checked out to be fine. I will get back to you all with what I find tomorrow (Nov. 18th) and hope to be able to say your all correct =o)I really really appreciate all of your experience and time spent posting here. Thank you all very much!!!
Replace the vacuum hose between the intake manifold and the booster…On rare occasion, it can collapse or the ply’s separate internally and block the hose, starving the booster for vacuum…But usually, this does not come and go…It’s special hose, be sure to use the correct stuff. Failing that, I would replace the booster…