Spongy Brakes after long use

Advice and help involving any mechanical issues.
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T1 Terry
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Re: Spongy Brakes after long use

Post by T1 Terry »

norman wrote:
Jon and Kay wrote:Ok, so I have a 10 tonne MH, Auto Tranny, four speed, Pacbrake that decreases revs by 500rpm, but auto tranny does not lock.

From Cooma to Bega, down Brown Mtn, using the pacbrake, all was good to the bottom (quite a long steep decent) until we reached the last 200m. The brake pedal went soft , for top half, then hard. Second push on pedal was hard to the top.Later when the brakes had not been used for a time, the brakes worked as usual, hard on first press.

Yesterday crossing Melbourne from Port Albert to Geelong , under brakes as stop and go on highway, all good until bottom of Westgate bridge, when again the brake pedal went soft on top half, then solid. Two presses and brakes were good to the top of pedal.

Brake fluid changed one year ago (dot 3).

What may be going on?
Like I said first up .... water in the brake fluid. water is not compressible.....
But steam is ;) The problem could be in either half of the split system, there is a valve that must see balanced pressure between both halves of the systems, if it does not the valve will be forced into a position that will block the half of the system with the least resistance, this stops the pedal going to the floor and half the brake system functions, a quick stab of the pedal will give a full pedal feel, but only half of the system will be operating.
When the brakes are not used for a while, the next operation of the pedal could shift the valve back to the central position and both halves of the system function.
The fact the pedal does not remain spongy after a partial failure makes water boiling to steam and causing 1 half of system to become in operative less likely to be the only problem, I think there is more than one problem.
The next issue is, the shuttle valve that protects against a total system failure because by one half of the system has a trip pin that should hold the shuttle valve against the failed system shut off and should cause a brake fail warning light to appear on the dash, this appears to be non functional and this should be rectified as soon as possible along with the full system flush and clean. This unit lives in the side of the master cyl, most commonly a pink colour plastic upper body with a 2 wire plug attached.
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Craig
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Re: Spongy Brakes after long use

Post by Craig »

You are correct Norman "water is not compressible.....". But when the brakes become hot after extended use the water turns to vapour (steam) and that is compressible. This is when you feel the sponginess in the brakes.
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norman
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Re: Spongy Brakes after long use

Post by norman »

So as I said in my first post. Change the fluid , it dont last forever. Jon has done quite a few ks in the year its been in there. Personally I would have changed it six months ago.Thats how you get pitted cylinders and other problems with moisture in there...
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Craig
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Re: Spongy Brakes after long use

Post by Craig »

I changed the fluid in my motorcycle clutch system to silicone fluid. It doesn't absorb moisture and therefore not cause corrosion problems. Or so they say. The bike has ABS brakes and it is not recommended for them. But it is a 10 year flush using silicone.
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Jon and Kay
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Re: Spongy Brakes after long use

Post by Jon and Kay »

BruceS wrote:Air over hydraulic like an old Bedford?
Hydraulics over hydraulic Bruce

Going to get a thorough check as although the brake fluid could contribute it may not be the only contributing factor.
Jon
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norman
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Re: Spongy Brakes after long use

Post by norman »

Jon and Kay wrote:
BruceS wrote:Air over hydraulic like an old Bedford?
Hydraulics over hydraulic Bruce

Going to get a thorough check as although the brake fluid could contribute it may not be the only contributing factor.
Driver error Jon ...the devil made me say that... :lol: :lol: :lol:
Better to remain silent and be thought a fool than speak and remove all doubt....
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Jon and Kay
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Re: Spongy Brakes after long use

Post by Jon and Kay »

norman wrote:
Jon and Kay wrote:
BruceS wrote:Air over hydraulic like an old Bedford?
Hydraulics over hydraulic Bruce

Going to get a thorough check as although the brake fluid could contribute it may not be the only contributing factor.
Driver error Jon ...the devil made me say that... :lol: :lol: :lol:
It's the other foot that is lead M8 :D
Jon
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Craig
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Re: Spongy Brakes after long use

Post by Craig »

any updates Jon?
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Jon and Kay
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Re: Spongy Brakes after long use

Post by Jon and Kay »

Bled the brakes.
We travelled from Port Albert to Werribee (accross Melbourne) without any repeat of the problem.
Looks like it was just some water in the brake cylinders, will replace when in Tassie in a couple of weeks.

See...the right answer is always the first one. :)
Jon
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norman
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Re: Spongy Brakes after long use

Post by norman »

Jon and Kay wrote:Bled the brakes.
We travelled from Port Albert to Werribee (accross Melbourne) without any repeat of the problem.
Looks like it was just some water in the brake cylinders, will replace when in Tassie in a couple of weeks.

See...the right answer is always the first one. :)
:lol: :lol: :lol: :lol:
Better to remain silent and be thought a fool than speak and remove all doubt....
If at first you dont succeed, Skydiving is not for you.

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