good evening gentlemen, i have a question about 3 wheeler brakes of which i am blissfully ignorant, its a fellow Morgan owners car, this is the problem, the brakes work perfectly.... then nothing, pedal goes all the way to the floor, switch off engine , restart engine brakes work normally. i have physically checked the system brake fluid is crystal clear level normal and no leaks.
any one have any ideas?

Thanks guys

Faulty master cylinder?

The problem cannot be related to the engine off/on as there is no connection such as a brake servo.

Hmmm, sounds ominous...,watching with interest ...

Have had almost the same problem with pedal going nearly to the floor. Have done the way home very slowly and put tt in the garage. Next day, I climbed on board and miracle everything was in order. My guess is that the brake sometimes stay to thight and as a result, oil is overheating. That's why the pedal went to the floor. When everything cools down it comes to normal again. The two routes to investgate for safety reasons are: the handbrake cable staying too thight or a stuck brake cylinder.

I have had this issue, as well, and it was resolved just as mysteriously. It has only happened once. I will say that it was a hot day and a long ride in some traffic when it happened. Although, by the same token, I have had rides like that before without issue.

If braked are used a lot then the fluid can overheat. Maybe a different fluid for your style of driving?

Brake fluid boiling should never occur on a street driven machine unless something is wrong. It takes extreme use, such as on a racetrack, or perhaps descending a long mountainside using the brakes the entire way, to cause fluid boiling and even there it's fairly rare. I've done hundreds of laps of racing events and track day driving in various machines and only once experienced pressure loss due to fluid boiling, and that was in a 6 hour enduro race.

If a M3W is boiling the fluid in normal street driving then one of the brakes must be dragging. Easy to check as when it occurs you can do a simple temperature check with your hand to find the offending brake as it will be extremely hot. Most likely the culprit is the rear brake, skulking as it does in the dark recesses of the back end where no air flows. On my car at least, the back brake seems to be somewhat variable in the amount of drag it evidences. Pushing my machine around in the garage I've noticed that if I push in reverse for some distance (like 20ft +) the rear brake will sometimes lock up, and I have to push forward to release it. (the machine is stone cold when this occurs) I've had the rear brake out twice now to clean and inspect it, and all appears fine. I suspect something is amiss with the auto adjusting mechanism, but I can't see it.

All this said, boiling fluid typically takes many minutes to cool enough to return to normal function especially if the car is stationary, so if the pedal goes to the floor then almost instantly fixes itself (like after an engine restart as described above) then that points more towards an internal issue in one of the master cylinders. Could be caused by several things like a deteriorating seal, a sticking piston cocking a bit and distorting a seal, congealed brake fluid distorting a seal, fluid contamination, etc.

I would start with a full and proper flush/replacement of the brake fluid and see what that yields in terms of information and results.

something to consider. All brakes will have a way for the fluid to return to the reservoir. On old gold wings we call it a spoog hole. These can be blocked by really small pieces of junk even if the fluid looks clear. If blocked the warming of the fluid will cause a pressure build up and the brakes will drag. This can lead to over heating of the brake fluid.

I had this happen on the front brakes of my '78 wing and also in the hydraulic clutch on our 86 1200 wing. In the first case the brake dragged. In the second case, the pressure build up disengaged the clutch.

When I used to work on industrial vehicles, I came across this problem a few times, what i found was that the brake pedal was not returning fully to replenish the master cylinder , if you physically return the pedal by putting your foot under the pedal the full brake pressure returns.

David Byrne.