Thread: Brakes Hot NO Pedal

Originally Posted by Tank

Peter and Graeme, Dave, Zook and Uncleho and all the others, thanks for the input, I was a truck mechanic before I stuffed my back and Hydraulic brakes are a distant memory, so if the fluid boiled and turned to Vapour, does that mean that the vapour returned to liquid state once cooled. There was no pedal pressure for a good ten minutes, so decided to drive in low range to get some air on the discs and calipers, even though the brakes were Hot they weren't excessively hot, some sizzling when spat on, but spit did stick and evaporate, usually overheated discs will throw spit (or any liquid) off like bouncing a ball off a wall, no discolouration or smoke or fumes. I am still puzzled by this problem and a lot of intelligent people have offered feasible scenarios for why it happened. I will strip the Whole system down and replace all rubbers and seals and whatever else needs replacing, I may also look at some vented rotors, is there any particular reason L/R used solid rotors, like they went out in the late 60's.
I do have a problem with my son's Toyota 4 Runner's brakes, you might be able to help, if you hit the brakes and keep the pressure constant the brakes work well, but if you release just a small amount of pressure (without removing foot from pedal) and then reapply pressure the pedal goes halfway to the floor, a pump or 2 and the pedal is ok, Have bled the brakes but doesn't make any difference, any Ideas would be appreciated, Regards Frank.

Moisture (water) in the fluid can vaporise then return to a liquid. It only needs to happen at 1 wheel and it will be impossible to build-up hydraulic pressure in the system. The fluid/steam wont get much hotter than boiling 'cause you no longer have brakes making it any hotter, so it doesn't take much time to cool back to just below boiling and hence water again.

The Toyota's problem is in all likelihood a master cylinder problem - soft rubbers and/or dirty/damaged bore.

Originally Posted by Blknight.aus

the brake gurus art work have discounted a leaking master cylinder... and are (without seeing it and only based on my description) leaning towards a failed vacume unit (which was my long shot since its come back ok) or boiled fluid in the calipers..



Im still backing my original hunch of boiled off in the calipers...

wouldnt a failed vacuum unit give you a pedal that wont go down.....(like my wife in a way....).....


as for boiled fluid.....i discounted this because he said they didnt get that hot......
and the fluid also wasnt that old.....

i didnt ask if the fluid was fresh and from a sealed container......nor if it was dot4.......


but if the brakes got that hot the pedal would have been spongy and needed to be pressed harder and harder......
the symptoms of brake fade......till there was no pedal..........and plenty of smell......

tank.......if you havent done anything else to the system.....remove the cap off the reservoir......
then go around and open the bleed nipples one at a time.....lock up each nipple before opening the next......
and see if clear fluid comes out......
or if it is aerated......


chances are if it got hot enough to boil there may still be some bubbles in the system......

oh...with the toymotas.....not sure which models though with my great memory.....
if the cap says use only dot3 fluid.....then use only dot3 fluid.....

apparently dot4 kills the seals prematurely.......



but dont ask me how or why........


so......when you fix the master cylinder.....keep this in mind.....

I'm voting for the master cylinder too.

I had exactly the same problem with my RR for around 3 years before it became too scary to drive off road.
On road it was fine, no matter how hard the brake were used. Take it off road for more than 30min or so, it would lose all brake pedal & I'd be relying on the handbrake.

I chased this for ages, checking hoses, checking calipers, bleeding the system over & over. Overhauling the master cylinder cured the problem.
There wasn't anything obvious wrong internally but it was bypassing somewhere.

Originally Posted by DEFENDERZOOK

wouldnt a failed vacuum unit give you a pedal that wont go down.....(like my wife in a way....).....


as for boiled fluid.....i discounted this because he said they didnt get that hot......
and the fluid also wasnt that old.....

i didnt ask if the fluid was fresh and from a sealed container......nor if it was dot4.......


but if the brakes got that hot the pedal would have been spongy and needed to be pressed harder and harder......
the symptoms of brake fade......till there was no pedal..........and plenty of smell......

tank.......if you havent done anything else to the system.....remove the cap off the reservoir......
then go around and open the bleed nipples one at a time.....lock up each nipple before opening the next......
and see if clear fluid comes out......
or if it is aerated......


chances are if it got hot enough to boil there may still be some bubbles in the system......

There is a way for the vac unit to fail that will cost you all of the brakes, But as its a complete physical failure of a component within the unit so far as I know unless there pigmy plastic welders in there It cant "come back" good its shot for ever and hsould be replaced..

if hes been out of vaccume and has the brake pedal on hard and then get gets some vaccume the pedal will sink (just like it does when your first startup) which is why I asked to confirm pedal position and travel.. hot discs without power assist in a rover are about as much help as most toyota drivers.

The heat thing depends on where he checked the brake temp..If he checked it at the resivior it would have been at engine bay temp, if he did the hand on the rim trick theres every chance that the rim wouldnt have had time to get hot but the pistons pushing on the pads would have had plenty of time to do so.

under normal driving conditions Id back you on the heat thing vs pedal action no problems...

but loong slow hard applications on a slow moving disc tend to put more heat into the brakes (being the caliper part not the disc) than the normal applications where the brakes are only on for a second or 2 at a time. This is more true when braking hard down slopes like some of us tend to get our rovers into. As the pad is in contact with the disc for longer there is more time for latent heat transfer and the pads, while excellent insulators arent as good as say the rentry tiles on a shuttle or racing brakes. I can easily concieve a 200m downhill braked crawl on technical but grippy surface easily taking the caliper temp over 130 degrees which is about the boiling point for 50/50 water/dot3. if youd been driving hard to get to the top (freeway run from 120ish to a halt at the top for hi-low change) before hand this then becomes really really easy.

to make this even more fun the watered brakefluid is denser than normal brake fluid so no points for guessing where that winds up.

all this time.......

Originally Posted by Tank

BTW it's an Auto,

Theres the problem, remove that and install a manual....



(i havent said that in oh a week or so now )

old thread i know, but Tank, what was the outcome?
I'm having the same trouble, except pedal not to the floor, just no brakes, this has happened 4 times in as many years on steep decents (all coming down from mt Hotham and Dargo. I.ve replaced, pads, rotors, fluid, 1 caliper, booster, over the last couple of years.
Brakes don't get smoking hot, only a slight hot brake smell
let it cool down and all is good again

(D1 auto)

Havn't followed this thread but my thought is the fluid(I see you have changed it) is getting hot.
I don't know if there is a higher temp. fluid available.

I find maximising the use of the gearbox when decending helps.