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break petal sinks to floor - but car stops

Posted: Tue Aug 13, 2019 11:13 am
by Jordanl17
I've had a long term issue with my breaks on my 2008 Mercury Sable Premier (150k). sometimes the break pedal goes almost all the way down to the floor, but the car does stop.. this can be recreated if you very gently press the pedal, then, with the same gentile pressure you can press all the way down. BUT if you are kind of hard on the breaks it will catch at the top of the pedal every time.
I had the car at the mechanic for a few weeks while he troubleshot it. I told him must be the master cylinder (I even bought one!), he said he installed it. still does the break sink. he never figured it out.
To this day, it still happens! what are all the possible causes for the break sinking to the floor (but only if you are soft on the breaks). thanks guys!

Re: break petal sinks to floor - but car stops

Posted: Mon Nov 11, 2019 7:57 pm
by 09Tbull
Hi, would make sure there are no leaks going on such as lines and calipers.
If that checks out you might want to have the ABS cycled with the shop Ford IDS or if you
have a laptop download version 2.3.15 of Forscan and do the pump style bleed yourself first.
Yes, it may be air trapped in the system from an improper bench bleed for the master or pocket of air within or around the HCU.
If you go through the manual bleeding process & the service bleed (computer based) and there is still an issue my answer would be a faulty HCU. That is the metal looking block that attaches with all the brake lines and has the black solenoid driver mated.
On many Ford vehicles this ongoing and often misdiagnosed issue. With age on any make of car these do fail as well. As mentioned by some this should have been a recall. Often times this issue happens after a ABS event and most of the time will never post a code. What happens is one or more of the internal valves can stick open allowing fluid to pass through and cause a disconcerting dropping pedal. My opinion on the actual cause may be a combination of weakening internal springs due to age/heat, possible fluid contamination, or deteriorated O-rings causing a poor sealing surface.
Another obvious concern is that the HCU is probably the most expensive part of the brake system besides the computer end of the system. A new one is well within the $800.00 range and that is hard to justify on a aged vehicle . The other option is used but who really knows the condition. If you go used try to find out the mileage on the used car and make sure the part numbers match up.
Good Luck!