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View Full Version : new develoment in my brake woes...



ddw350
05-31-2004, 12:17 AM
i fooled with it for awhile today and did as one member suggested by listening for a vaccumm leak under the dash where the booster enters through the fire wall.it wasnt till i pushed the pedal down with my hand and noticed a "swooshing" sound from the center[where the rod is surrounded by a yellowish foam,i took the grey boot off]of the booster.i covered this area with my hand and noticed air pushing[or pulling,i cant really tell] from this area.its like when i hit the brake pedal at idle,the booster just blows the air right out the back of it,causing my pedal to go way down...?? in case you didnt know,my brake pedal goes way down with no resistance at idle or low rpm,s.it then triggers the firm ride light.i put a new master cyl on when i noticed brake fluid dripping on my floormat,[it ran down the push rod from the leaky master cyl i guess.but at regular speeds the braking is fine] then the whole system was vaccumm bled to ensure no air in the system...

MercsSC
05-31-2004, 08:16 AM
brake booster is a real simple design...just a big diafram..negative pressure on one side [vacume] and normal pressure on the peddle side ...the negative pressure makes it easy to press the peddle down..the swooshing sound is the air moving in and out of the brake booster ..and is normal....and is usally a good sound ...meaning the diafram doesn't have a hole in it ...leaking and balancing the pressure inside the brake booster.....that being said the peddle... should be hard as a rock... to move after a inch or so of brake peddle travel ..and after changing your master cyclinder ...you need to totally bleed your brakes ..sure sounds like air in the system.....and after a few pumps of the brake peddle ...you build some brake pressure and vacume starts to work for you again ..brake light coming on is a result of the too much brake travel... and is trigging the switch..dave

thought I would add ..vacume bleeding is not the way to do these brakes...should use the pump to prime the system ...key on .. foot to floor open rear calipair one at a time ...need 2 people ....the reason being you can't bleed the pump in the system ..

ddw350
05-31-2004, 01:43 PM
ive also heard that vaccumm bleeding is not the wayto do thesebrakes[after we did it of course] i thougt i was on to something when i heard and felt the swooshing at the booster...damn...anyhow any ideas on why this suddenly happened..one day brakes are fine,next thepedal goes way downat idle .i noticed it in park actually.im taking to a shop on wed,they do a pretty good job with brakes,so maybe they can pinpoint the problem,i'll continue t tinker with it till then....

MercsSC
05-31-2004, 03:33 PM
ive also heard that vaccumm bleeding is not the wayto do thesebrakes[after we did it of course] i thougt i was on to something when i heard and felt the swooshing at the booster...damn...anyhow any ideas on why this suddenly happened..one day brakes are fine,next thepedal goes way downat idle .i noticed it in park actually.im taking to a shop on wed,they do a pretty good job with brakes,so maybe they can pinpoint the problem,i'll continue t tinker with it till then....

again I think it is air in the system ..and likely in the HCU [hydralic control unit] cause you are not building hydralic pressure...now either the pump isn't coming on at startup ...or there is air in the pump/HCU preventing the build up of pressure...I have 1st generation Tbird ...which is totally different
than 94-95s and don't have a very good break down ..on how the system works ...dave

Digitalchaos
05-31-2004, 06:35 PM
ddw350; I agree with Merc and also think its air in your lines. I have been having a problem similar to yours. See this thread (http://p075.ezboard.com/fthunderbirdundergroundfrm2.showMessage?topicID=10 02.topic) for some excellent information on the '93 - '95 braking symptom and how these symptoms might come about.

There are only so many things which can cause a weak peddle. Air is the most common, and requires a special tool to properly bleed on Teves Mark IV. Other culprits could be a stuck caliper, a bad rubber section of brake line which bleeds fluid into the outter rubber layer alleviating pressure, or a bad master cylinder.

You can eliminate the master cylinder as a possibility as you just changed it. If it were bad, it would leak (as yours did) and it would not hold the car at a stop.

What year is your SC?

ddw350
05-31-2004, 09:35 PM
thanks for the article digitalchaos....i have a 94sc,i dont understand how this suddenly happened....i didnt relize the abs unit has a bleeder also,hell i never even heard of a HCU and ive owned this car for 8 years.is it possible to bleed at the HCU,or do you have to do it at the abs unit? im trying to get enough info so i can tell the mechanic i know what going on in a more detailed manner...so this is a teves mark IV system?..and requires a special bleeding procedure [and tool] at the HCU/or abs system through a valve at the abs unit itself.also i'll mention this car has 2 seperate brake systems,1 main one that can be bled from master cyl to the calipers,the 2nd which is the abs system that has a valve seperating it from the main system,which can also get air in it...where did the air come from???