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non returning brake pedal - how do brakes work?
Posted: Mon Oct 10, 2005 9:14 pm
by JJbRacing
this is a new one on me.
brakes work great when applied on my new 325cab - problem is when you lift of the brake pedal it stays down!
it does slowly return but it needs a tap from underneath with the foot to get it up ;)
the cars been sitting in a barn for a couple of years so i've lubed up the pedal mechanism from pedal to connector to master cylinder and all feels pretty free.... so could it be the hydrolic system holding it in?
soooo - thats where i run out of knowledge etc. why does a brake pedal return? where does the pressure come from? the pedal has a spring on it and thats looking ok - but its a bit puny. so how do brakes work
cheers for any advice - track day in two weeks and i need to get her ship shape!
ta!
jez.
Posted: Mon Oct 10, 2005 9:26 pm
by orangecurry
I'm sure someone will be along with a good explanation of how brakes work (big question), but if the car has been sitting for a long time, the seals in the brake master cylinder will have perished, allowing fluid to go past as you push on the pedal and so push the rod into the cylinder
capiche?
Posted: Mon Oct 10, 2005 9:33 pm
by Simon
Pretty sure there's a large spring connected to the pedal, as with the clutch pedal?
Posted: Mon Oct 10, 2005 9:34 pm
by JJbRacing
ya.
but what i cant fathom is why the brakes work so well! also the fluid level stays true! just no return of pedal
Posted: Mon Oct 10, 2005 9:38 pm
by orangecurry
ah you didn't say that!
if the brakes work well it must be the... BrianMoooore, thank GOD you've arrived!
Posted: Mon Oct 10, 2005 9:40 pm
by JJbRacing
there is a big spring - but its all connected and looks ok
good old "how stuff works" has some info... but still not understanding the return pressure...
http://auto.howstuffworks.com/master-brake1.htm
Posted: Mon Oct 10, 2005 9:46 pm
by JJbRacing
Posted: Mon Oct 10, 2005 9:51 pm
by JJbRacing
ok - so based on the pics on how stuff works it seems the master cylinder has a couple of springs in it.
now the car has lovely clean fluid in it - so im thinking that maybe it lost fluid in storage, the MC got all coroded, but prev owner just toped it up when it came oua storage.
i have a couple of spare MC's kicking around so maybe a swap is in order?
Posted: Mon Oct 10, 2005 9:58 pm
by orangecurry
ok ok so when I said 'ah you didn't say that' I meant 'I didn't see that'
If you've got the spare MCs and the time I would swop them over - I had a non-servo race car which after not being used for a while still stopped well but had no return on the pedal, but I assumed that was the pedal mechanism (which you said was free-moving on your car) but it might have been the MC
I guess we are talking very small movements of brake fluid volume here
Posted: Mon Oct 10, 2005 10:08 pm
by Andy_magic
In a brake master cylinder you have a piston inside the cylinder, one side has a rod that connects to the brake pedal, the other side of the piston has brake fluid that pushes the calipers open.
If you leave a motor standing for ages you can get corrosion in the master cylinder on the side that the pedal connects to, hence when you push the brake pedal, all works well, but when you realease the pedal the springs (Normally 1 inside the MC aswell) can't push against the corrosion and you have sticky pedal.
Posted: Mon Oct 10, 2005 11:11 pm
by johnna
Swap the M/C or get a seal rebuild kit. It is the back pressure seal that is goosed.
