23 replies to this topic

[Elwyn]

Excellent video. I like slow and methodical. And simple. Many thanks again.

[Revd]

What happens when you gravity feed yellow tag master cylinder - there is next to no head of fluid above the upper outlet?

[Tornado99]

What happens when you gravity feed yellow tag master cylinder - there is next to no head of fluid above the upper outlet?

Since you're only letting the bleeder valve flow a drop at a time, you should be fine if you just keep a closer eye on the level as you do each wheel. Must be a bigger issue if you are using someone to pump the brake as that can move a lot more fluid instantly. 

 

In the video I posted, note one of the advantages of the gravity bleed, esp for older cars, is that you never need to move the brake pedal, and push it past its normal movement range like with other methods. This means the MC piston never gets pushed into the unused area of the MC bore, where all the nasty stuff lives and the bore walls can be rough...thus damaging the piston seals. 

 

Only drawback is that it does take a bit longer to perform. 

Edited by Tornado99, 11 September 2020 - 07:54 PM.

[Revd]

Surely there needs to be enough weight of fluid to move any air locks or bubbles, especially in a dry system. You may run the risk of displacing fluid by drawing air in through the bleed nipple threads. I’m just not convinced there is enough fluid in the reservoir above the outlet - happy though to be corrected by anyone who has successfully gravity bled a yellow tag circuit

[Tornado99]

Surely there needs to be enough weight of fluid to move any air locks or bubbles, especially in a dry system. You may run the risk of displacing fluid by drawing air in through the bleed nipple threads. I’m just not convinced there is enough fluid in the reservoir above the outlet - happy though to be corrected by anyone who has successfully gravity bled a yellow tag circuit

The reservoir sits much higher up than the brake bleed valve at the wheels....it is that difference in height which gives the flow out the bleeder. It doesn't matter how much fluid is held above the port on the MC, as long as there is some amount to prevent air being drawn down the port.

[Revd]

Sorry, I’m probably being a bit dense but surely for fluid to flow through the top outlet port requires either a head of fluid or positive pressure from pumping the pedal/ eezibleed otherwise your are just draining the system through the bleeder. It is quite possible that a very small head (say 10mm if brimmed, because that is difference in height between rim of the reservoir and the top outlet) of fluid is sufficient and I’m unduly pessimistic.

[KTS]

Sorry, I’m probably being a bit dense but surely for fluid to flow through the top outlet port requires either a head of fluid or positive pressure from pumping the pedal/ eezibleed otherwise your are just draining the system through the bleeder. It is quite possible that a very small head (say 10mm if brimmed, because that is difference in height between rim of the reservoir and the top outlet) of fluid is sufficient and I’m unduly pessimistic.

The 'head' is the height difference between the fluid level in the reservoir and the bleed nipple. Even if the top outlet on the master was higher than the reservoir the fluid would still drain (..assuming seals etc would allow). No different to siphoning fuel out of a tank.

[Tornado99]

Sorry, I’m probably being a bit dense but surely for fluid to flow through the top outlet port requires either a head of fluid or positive pressure from pumping the pedal/ eezibleed otherwise your are just draining the system through the bleeder. It is quite possible that a very small head (say 10mm if brimmed, because that is difference in height between rim of the reservoir and the top outlet) of fluid is sufficient and I’m unduly pessimistic.

Any fluid above the port is sufficient to produce flow down through the upper port. You've only got to keep some volume there for it to continue. So you say 10mm (in depth?), it's whatever volume is sitting above the port that determines how much can flow out. Keep upper port under fluid and it will keep dripping from the bleeder at the wheel. If you are concerned about air getting into the line at the bleeder valve, you can run the clear hose so it makes a vertical bend higher than the bleeder...forms a fluid trap as drips flow out. Keeps air away from bleeder opening. If valve threads are of concern for air leaks, put a dab of grease around them before opening. I have not found that is a concern when I open the valve 1/2-3/4 turn only. 

[Revd]

Might well give it a try next time. I used an eezibleed when I replaced callipers and master cylinder etc during lockdown so not planning to revisit brake bleeding (my least favourite job on a mini) just yet