Brake Pedal Goes to the Floor?

Tiny
JRUSHER
  • MEMBER
  • 1969 MERCURY COUGAR
  • 5.8L
  • V8
  • 2WD
  • AUTOMATIC
  • 131,000 MILES
After restoring my car listed above, I tested the brakes and found that with the engine off, the brake pedal was firm, but when running, the pedal sinks to the floor. This car has power disc/drum brakes.
I have done the following:
1. Replaced front pads and rotors, verified calipers are on the correct side.
2. Replaced master cylinder (old one had rust)
3. Booster (Bendix) was rebuilt by Booster Exchange
4. Replaced Distribution block/proportioning valve (incorrect unit installed by PO, and the shuttle was frozen)
5. Bench bled MC
6. Flushed all brake lines
7. Set booster rod gap to.020" (mfg spec)
8. Bled system 3 times
9. Confirmed no leaks
10 verified engine vacuum (18 Hg) at booster
11. Verified check valve in booster was OK
12. Adjusted rear drums for a slight drag
13. Performed drain-down test on booster (pump pedal 3 times, start engine, pedal goes to floor = FAIL)

Based on the results of item 13, it appears that the booster is still bad. What do you think? Are there any other tests I should perform?
Thursday, July 17th, 2025 AT 6:43 PM

11 Replies

Tiny
STEVE W.
  • MECHANIC
  • 15,233 POSTS
It sounds like the vacuum valve in the booster is leaking. Disconnect the vacuum line to the booster. Try the brakes. They should be hard and no real pedal travel. Now reconnect the booster and start the engine, pedal sinks? Are the brakes applied while the pedal sinks?
You pretty much have it narrowed down.
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Friday, July 18th, 2025 AT 8:59 AM
Tiny
JRUSHER
  • MEMBER
  • 13 POSTS
Hi Steve,
Thanks for your reply. I disconnected the vacuum line, brake was firm with about 1" of pedal travel with engine running (vac line plugged), and same with engine off. When vacuum was reconnected pedal goes to floor with very little effort. I also tested the valve by removing it and blowing into it in both directions, and it seemed OK, ie flow in only 1 direction.
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Friday, July 18th, 2025 AT 11:09 AM
Tiny
JRUSHER
  • MEMBER
  • 13 POSTS
I replaced the check valve and still have the same symptoms - let me know if you have any other suggestions.
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Friday, July 18th, 2025 AT 1:31 PM
Tiny
JRUSHER
  • MEMBER
  • 13 POSTS
Just to be clear, are you talking about the Control Valve inside the booster, or the check valve that the vacuum line connects to? I have assumed the latter.
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Friday, July 18th, 2025 AT 1:35 PM
Tiny
STEVE W.
  • MECHANIC
  • 15,233 POSTS
There is a control valve inside the booster that responds to the pressure on the pedal. It is what opens to allow vacuum into the diaphragm and gives you the boost. If it is open and the vent side is stuck the pedal would go down but not come up until you released the vacuum. If only the transfer valve section is open the vacuum would apply and then it would come back up. The valve on the outside just traps vacuum in the booster if engine vacuum drops.
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Friday, July 18th, 2025 AT 1:40 PM
Tiny
JRUSHER
  • MEMBER
  • 13 POSTS
OK thanks. I repeated the test that you suggested and results were the same. After I disconnected the vacuum hose the brake pedal moved about an inch when I applied pressure, but when I pressed really hard, the pedal went down further. When I reconnected the hose and started the engine, the pedal did not move until I applied pressure, then it went to the floor. Are you still thinking the booster is the issue, or could I have more than one problem?
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Friday, July 18th, 2025 AT 1:54 PM
Tiny
STEVE W.
  • MECHANIC
  • 15,233 POSTS
It may go down but if the system is sealed it will stop, They you are dealing with things like line expansion and bracket flex. You might want someone to watch the rubber lines as you test, it's rare but one could be ballooning out from the pressure. Other than that the next odd item could be the seals in the master are bypassing. The thing is that it doesn't do it without the booster.
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Friday, July 18th, 2025 AT 4:03 PM
Tiny
JRUSHER
  • MEMBER
  • 13 POSTS
I tested the MC by removing the lines and plugging the outlets. I applied brake pressure and the pedal would not move, both with engine on and off. I will check the rubber lines as you suggest.
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Friday, July 18th, 2025 AT 4:18 PM
Tiny
STEVE W.
  • MECHANIC
  • 15,233 POSTS
Other than something strange like the rear shoes flexing or the lines there isn't anything past the testing you have done. Look and listen at the brakes while someone applies them. That era was bad for serious over boost of the brakes because Ford wanted to target more women and tried to make the Cougar a "softer" car than the Mustang. Even then this isn't normal.
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Saturday, July 19th, 2025 AT 5:53 AM
Tiny
JRUSHER
  • MEMBER
  • 13 POSTS
I checked all the flexible hoses while having someone apply the brakes, and did not see any bulging or collapsing. The front lines appear to be fairly new, the rears are older but appear to be in good shape. I also watched and listened to the brakes while they were applied. The front calipers had very little movement as the pads are new, but I was not able to rotate the wheels with the brakes applied. The rears were audible, and I pulled the drums off to observe the movement, and the shoes expanded and retracted normally. I guess that leaves the booster - I will replace it and let you know how it goes.
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Saturday, July 19th, 2025 AT 8:59 AM
Tiny
STEVE W.
  • MECHANIC
  • 15,233 POSTS
We'll be here. With everything you tested there really can't be anything else.
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Saturday, July 19th, 2025 AT 12:17 PM

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