View Full Version : Soft Brake Pedal

Thomas A
12-05-2016, 06:28 PM
Wondering if I could get some thoughts on next steps to resolve my ongoing brake issue.

1997 Thunderbird 4.6L SC
4 Wheel Disc, No ABS

Symptom: Soft pedal, weak brakes
Replaced: Master Cylinder, all calipers, rotors, and pads
(verified bleeder valves are pointing up on calipers)

I bought this car having terrible brakes; Decided to go through and replace major components (upgraded to PBR on front) and issue has not changed. The MC was bench bleed, and I have run what seems like a gallon of new fluid through the system bleeding them. They have shown some improvement, but not much. I have inspected everything and see no leaks or fluid loss.

Last night after bleeding the front brakes yet again, the car for whatever reason wouldn't start. It cranked but wouldn't fire. I wiggled the connector to the CPS and it fired up. Backing out of the garage the brakes were awesome; grabbed very quick and hard. By the time I moved the car around and used the brakes a few more times, they were right back to their soft, awful status.

Wondering if this is a vacuum related issue? Perhaps the additional cranking of the engine produced enough vacuum and once it was up and running, it was gone again. The pedal isn't hard, so the booster seems to be working just fine. Also seems to hold vacuum very well (pull off vac hose hours after engine is off and still makes a large wooshing sound). My SC had a big cam (not enough vacuum) and the feel was completely different. It would go from good brakes to none with a hard pedal when the vacuum ran out. This is just consistently soft, poor performance, so not sure vacuum issue makes sense.

Any ideas what else to check. Why would they be so soft, then perfect for a couple of applications right after bleeding last night, and back to soft? Any insight would be appreciated.

12-05-2016, 06:57 PM
It's not uncommon for aged brake hoses to turn into balloons under pressure, giving a soft feel. They can also swell up inside, making it hard to bleed reliably, etc.

Might be time for new all around. I vote braided if the budget won't bust.

If it was vacuum issue, that might mean hard pedal, not soft? What does it feel like if you let it sit with vacuum to the booster disconnected, then fire it up and test the pedal, then connect and re-test?

I like to vacuum bleed, BTW.


Thomas A
12-05-2016, 07:13 PM
Thanks for the reply; I agree that a vacuum issue should cause a hard pedal rather than a soft. And all indications seem to suggest that the booster is still functioning as it should.

One other thing that I noticed last night. On the front left caliper, the fluid initially came out dark (all fluid has been replaced in system), then started running clear. I thought perhaps the caliper had a major air bubble in it that trapped some old fluid, and when the brakes were all of a sudden working perfectly once I was done, that I had solved the problem. When my success was short lived, I ran out of ideas. What happened last night would seem to rule out brake lines as that should be more consistent of an issue, in my mind. I'm sure replacing them wouldn't hurt, but not sure if it would fix this problem.

Yup, I use a vacuum bleeder myself. Started at the right rear, then left rear, right front, and left front. I pulled 6-8 oz of fluid through each caliper. I guess with replacing so many items, there could still be air in the system? I've bled the brakes 3-4 times at this point :S

12-05-2016, 07:20 PM
Might be worth one more shot today, to see if you get anything after letting it sit overnite.

Next time they go soft, see if you can have a helper replicate while you study the lines in action.

I bought this car having terrible brakes

...not liking that... Any chance the firewall is cracked and the master cylinder is causing it to flex badly? Is the master cylinder all stock configuration? Is the parking brake operation smooth/reliable?


Thomas A
12-05-2016, 07:27 PM
I'll be driving it to work this week, but might give bleeding it one more shot this weekend. Can't hurt other than time and more brake fluid :S

The soft pedal is consistent. It hasn't changed since I bought the car, replaced the parts, or bled it, other than last night for the first couple stops, then back its consistent state. Everything is stock setup other than the Mustang PBR calipers that I added. The only thing on those that I noticed is that one bleeder screw threaded into the caliper further than the other (they were reman units). The both bleed perfectly fine, so not sure if that is a concern or just a difference in the manufacturer process. Parking brake seems to move fine (cable were free when I changed the calipers), although I'm not sure I actually have tried it. Car has 102k miles and seems to be fairly solid otherwise.

sam jones
12-05-2016, 08:26 PM
Good evening

There might be a problem the the brake booster and or check valve. Brake booster failing for soft pedal is on the low side. Next time the "sponge" condition happen listen for an air leak/"swishing sound" when you press and release the brake pedal. If this sound is identified replace the brake booster check valve first. If the sound still repeats suspect the brake booster.

See link below:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwjn8uizot7QAhVCilQKHcw0B9MQFggpMAI&url=https%3A%2F%2Fwww.yourmechanic.com%2Farticle%2 Fsymptoms-of-a-bad-or-failing-vacuum-brake-booster-check-valve&usg=AFQjCNE2L3xz4RuwQj_W6G691zGzUfYAvw&sig2=MeqMoWpv653IIpQWIsjkBg

Good Luck.

Thomas A
12-06-2016, 12:48 AM
Thanks; I have been listening for that very sound as I have heard it on another Thunderbird in the past; none that I have detected on this one; I may swap the check valve with a known good one this weekend as its not difficult to do. I'll report back what I find out.