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too much brake, why??

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too much brake, why??

Postby ezytiger83 » Wed Sep 09, 2009 10:44 am

I'm having no joy elsewhere, and before i take my MU to a brake specialist I thought i'd ask here.

Problem is my brake pedal is building up too much pressure. I have a basic understanding of the vacuum requirement, so if i had to guess, i'd say pressure keeps building up and however it is released is failing. at start up the brakes are normal, but the longer i drive (and presumably, the more i'm on and off the pedal), the brake pedal hardens up to the point that i only need apply a few mm of movement to the pedal to achieve full brake. i also feel that the brakes may be riding slightly at this stage as well, as zippiness is gone, feels like i'm driving up 45 incline. (reference: like driving up to high range in townsville)

the MU came with a few mods when i got it, i bought MU wholesale, so i never spoke to the previous owner. could he have been in the middle of a mod? the brake booster looks brand new, the EGR was already gone and I've since ditched the vacuum hose bird nest. at the moment, being the dodgy DIYer that i am, i've tapped retic into the vacuum hose at the brake booster and run it into the cabin with an adjustable drip head which i ended up leaving wide open constantly. dodge setup helps slightly, but the 4mm hose doesn't let it out at quite the same speed vacuum builds up.

i would love to just tap a vacuum regulator into the line, it sounds simple in theory, but i can't even find a guage that registers negative pressure (plenty of adjustable positive pressure regulators, and no you can't just put them on backwards) and then i would need to monkey with it to find the right point to release vacuum.

whew :? there you go, any help or should i go ahead and book a pro? hopefully one of you loves puzzles :lol: personally, i've fried my brain thinking about it :roll:
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Re: too much brake, why??

Postby pig75 » Wed Sep 09, 2009 1:48 pm

I don't think it's a vacuum problem. It sounds like a dragging break to me. When cold jack up each wheel and check they spin free. It might be a bit hard to tell if it has a LSD in the rear then take it for a drive till it gets hard then check the wheels again. My bet would be the rear are dragging
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Re: too much brake, why??

Postby geeves » Wed Sep 09, 2009 2:22 pm

Im with pig75 on this one but a second test would be to drive it till the fault is occuring then stop and check all the hubs for heat. The one that burns your hand is the faulty one.
My money is on sticky calliper slides or siezing piston
Sanding your knuckles before starting work can help. That way you cant skin them
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Re: too much brake, why??

Postby slightlydodgy » Wed Sep 09, 2009 5:32 pm

When checking for heat DONT touch the disk I found out the hard way with an old HQ with a sticky master cylinder smelt cooking steak and lost sense of feel in those fingers for a long time
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Re: too much brake, why??

Postby geeves » Wed Sep 09, 2009 8:09 pm

You only need to touch the wheel but even that can burn. Even a fast run through a winding hill road can have the rims too hot to touch.
Sanding your knuckles before starting work can help. That way you cant skin them
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Re: too much brake, why??

Postby ONO2HI » Wed Sep 09, 2009 9:03 pm

just a side note, i had had similar issues you stated with vacume, my light would come on after 1-2 pumps, after pricing a new alternator i decided to investingate my set up, i stripped it down greased everything up and got rid of all the old vacume hoses and only ran the nessary and it was mint. just my 0.02cents
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Re: too much brake, why??

Postby MattMu » Wed Sep 09, 2009 9:07 pm

Find your problem first before you start messing with vaccum etc.

It can be the booster (vac) so stop just bleeding vac out of it, find out if that IS your problem...disconnect and block the vac hose going to it and see if the pedal returns to normal...but just heaps more effort.

The booster has a reaction disc inside that transmits pedal feel to the driver, otherwise the booster is all or nothing, rather than progressive. So if it improves disconnected but on/off once conected then you have the problem.

Otherwise it could be other problems.

Is it the brakes dragging....jack up wheels after a drive and turn them...turn ok...not dragging. If dragging...is it all, just front or rear, or just one.

If one-siezed caliper or slides.
If it front/rear- mastercylinder or proportioning valve fault.
If all then m/cyl or booster adjustment incorrect.
I would suspect the compensating port inside the master cylinder is staying covered. No problem at first but as the brakes heat up the fluid expands and holds pressure on the brakes. Need to check pedal height, pushrod length.

There is no pressure in the booster...only all vaccum on both sides of the booster or vaccum on one side and various stages of atmospheric pressure on the other side. This pressure differential (low and not-so-low) creates force towards the mastercylinder. If the booster is always creating a mechanical advantage then the vaccum port is closed and the atmosheric port is remaining open.

The factt he booster is new...says the problem was present before the sale and they tried to sort it without success, so I doubt the booster is the problem.
Brakes are not to be messed with...if this is beyond your skills or training get it to a mechanic...bleeding vac of is unneccessary, dangerous and likely to cause an accident.

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