Spongy Brakes after long use

[Jon and Kay]

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?

[RussellB[SA]]

I'm not a mechanic, but the last time this type of thing happened to me it turned out to be a failing master cylinder .

Hope it's not what is happening to you.

Regards

[norman]

Jon, Change the brake fluid before you proceed with anything else. Sounds like its had its day. Moisture heat etc....

[Jon and Kay]

Thanks guys, more info. We have disk brakes all round. Split system...front/rear. I'll inspect all hoses and cylinders. I was guessing that a fluid line was expanding (just front or rear) when brakes got hot after extended use. Browns Hill is about 1/2 hour of extended braking, and the freeway was stop and go for an hour. The unusual thing is that except for these two occasions I haven't experienced any issue. I did an emergency brake the other day just to be sure that nothing was about to break.
p.s. I've got a mate who says he thinks he's addicted to brake fluid. But it doesn't trouble him, he says he can stop any time.

[Newcastle George]

Seems like master cylinder problem to me. I had similar with air/hydraulic clutch slave cylinder, fitted new kit, problem solved.

George

[Craig]

Hi Jon,

It sounds like you need to do a flush. The fluid is hygroscopic (absorbs moisture) and after extended use the moisture boils and this is when you feel the spongy feeling in the pedal.

Craig

[BruceS]

Air over hydraulic like an old Bedford?

[supersparky]

Hi Jon,

It sounds like you need to do a flush. The fluid is hygroscopic (absorbs moisture) and after extended use the moisture boils and this is when you feel the spongy feeling in the pedal.

Craig

I reckon Craig is on the money here.

[jon_d]

The brake pedal went soft , for top half, then hard. Second push on pedal was hard to the top.

Don't discount the master cylinder.

A good test is to gently press on the pedal for 5/10 minutes while stationary.
If its leaky, the pedal will slowly sink.
If its good, it won't sink because the fluid is not slipping past the seals.

[norman]

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.....