Talk Morgan Forums Morgan Sports Cars Morgan Three Wheeler & Super 3 M3W brake failure

Agreed. Brake fade is the term most often associated with overheating and outgassing of the brake friction material leading to a minicule gas layer between friction material and disk or drum. With brake fade, the pedal typically remains "high and hard", but retardation ceases. Not at all the same as fluid boiling.

As trucking-on commented, fluid boiling results in a temporary but dramatic increase in gaseous water in the line with the pedal firmness going to near zero until the fluid cools down. Certainly the braking performance "fades" with fluid boiling, but this is not the common usage of the term fade.

I insisted the dealer change the fluid on my mustang at 4 years. They say they test it & only change when necessary. They are only testing what's in the reservoir not the fluid in the pistons/calipers.

I would agree with the brakes overheating and causing the brake fluid (water content?) to boil and allow the pedal to go full travel. The brakes overheating could be caused by poor self adjustment, corrosion in the wheel cylinder sticking the pistons or maybe not letting the handbrake off fully? If they got hot enough to melt the dust cap they must have got very hot at some point.

I have Speed Bleed NRV nipples on the brakes that make bleeding and changing the brake fluid much easier. It gets replaced every two years as most brake fluids absorb water over time. If you are checking the brake shoes remember that the friction material thickness on one is normally less than the other, even from new.

probably the most underperformed necessary maintenance. You do need to factor in the humidity where you live as well as DOT 3 or 4 fluid is hydrophilic and will absorb moisture which leads to lowering of the performance of the fluid.

My understanding of gasses is that they expand 1000 or more times their own volume per degree centigrade rise in temperature and that the danger is that this causes the brakes to jam on suddenly with disastrous results - or am I missing something??

Yes, I am missing the point that when the pedal is at the top of its travel the excess volume can pass to the reservoir but surely if the pedal is depressed the vapour pressure will push it back up till the pressure can be vented back to the reservoir? I do not understand how the expanding gasses allow the pedal to travel to the floor.

Two years and 12k miles on my Trad resulted in black fluid in the rear circuit, and badly discoloured fluid in the front. Changing fluid is cheap, and I'm amazed that a dealer would say that. Internal corrosion is more about aging than mileage.

I always refresh at two year intervals.

We call it a spoog hole in older gold wings. The place where the pressure should relieve up into the master cylinder. I have seen in older bikes where this hole becomes blocked and if it does the excess pressure will indeed start to lock up the brakes. I had that happen on my '78 wing, and also on the clutch on our 1200 (in which case the pressure disengaged the clutch). In the either case it didn't so much "jam" as get tighter and tighter (or in the case of the clutch, looser and looser)

While the gases do expand with temperature they are also compressible, while fluids (generally) are not. So the mechanical force from the foot/pedal is greater than the expansion force of the heated gas, and the pedal goes 'squish' until all the forces are balanced.

+1 it sounds very like classic boiling fluid brought on by whatever caused the rear brake binding on and being run at speed like that

I think I have a handle on it now although not a way of fixing the problem. Thanks guys for your input it has helped me see the wood amongst the trees.

It seems that when the rear brake gets hot due to prolonged braking down hill ( my experience ) or if the handbrake has been left partially applied, the drum expands allowing the auto adjuster to go one notch further than normal - it is a very fine ratchet adjuster. This keeps the brake applied generating more heat - gassing off any moisture in the fluid and giving rise to the pedal going to the floor. The only remedy seems to be removing the back wheel, levering the over-tight shoes and brake plate out of the drum, re setting the adjuster and putting it all back together. Change the fluid just in case it has deteriorated and carry on as normal. Short of changing the pitch of the adjuster ratchet I see no obvious way of avoiding this annoying and possibly dangerous glitch from recurring.