Bench bleed BMC on car?

[Spetz]

Hi guys,

I'll be replacing my BMC shortly and have been watching youtube videos on bench bleeding.

I'm wondering if I get a BMC bench bleed kit that runs the hoses back into the brake fluid reservoir is there any reason I couldn't do this while the BMC is install on the car?
ie, run the hoses to the reservoir and keep pumping the brake?

I do not have a vice to hold the BMC and doing the above would be much easier for me if it works.

 

[Trav]

Sure just have someone press the brake pedal very slowly a few time till the air is out, remove the bleeder lines the connect the cars lines.

 

[eljefino]

Do it on the bench *if the instructions with your M/C say you need to*. When it's on the car the "far end" away from the pushrod is a high spot and you won't get the bubble out. You don't need a vise, just hold it in your hands and push firmly with your thumb(s). It'll go, slowly.

That's why they include the stupid plastic hose and adapter thingies and instructions *on your particular M/C* -- they wouldn't if "normal" bleeding did it.

 

[Spetz]

Thanks guys,
I am going up a size in BMC and as such getting a used unit so no kit with it

 

[E150GT]

I "bench" bled my master cylinder pushing the MC against the wall and using a screw driver to push the cylinder in. Worked for me.

 

[Trav]

Originally Posted By: eljefino
Do it on the bench *if the instructions with your M/C say you need to*. When it's on the car the "far end" away from the pushrod is a high spot and you won't get the bubble out. You don't need a vise, just hold it in your hands and push firmly with your thumb(s). It'll go, slowly.

That's why they include the stupid plastic hose and adapter thingies and instructions *on your particular M/C* -- they wouldn't if "normal" bleeding did it.


Not talking about "normal bleeding". You can use a bleed kit for $7 for AZ. Its a 2 min job once its bolted in, it no different than being in a vise, its just another way to hold it.

http://shop.advanceautoparts.com/p/dorma...CFRJYDQod0uUBOQ

 

[eljefino]

Originally Posted By: Spetz
Thanks guys,
I am going up a size in BMC and as such getting a used unit so no kit with it


If the firewall tilts back and the "far (forward) end" of the MC is the high spot and the top of the "far end" isn't tapped for a line you'd want to bench bleed.

 

[Trav]

The OP can do the one on his car if it was near vertical.





But in any case that's not how it works. I have run into this a bunch of times over the years that someone has let the MC go dry when changing brake fluid and on some cars getting the MC off to bench bleed it can be time consuming. Look at the picture, notice there are 2 reservoir ports for each piston.
When you use a MC bleeder hose you run the hoses into a filled reservoir so they are always covered in fluid. When the piston is depressed the fluid goes out of both outlet lines along with it the air but the next part is where I think you mean the problem is.
When the piston is released any small amount of trapped air before the primary piston will be drawn back and escape through the first orifice going to the reservoir. Granted it might take a few more depressions but its coming out.

This is the reason you see air bubbles coming out of both the hose and the hole in the reservoir, the small angle the MC may be on is of little consequence but if it is of that much concern just put the rear end on ramps.
The only drawback to doing it this way is it turns it into a 2 man job. Note it can take as much as 20 or even more depressions to rid the MC of air regardless if you bleed it on or off the car.

 

[artificialist]

The reason some people bench bleed master cylinders is because it is easy to push the piston too far and damage the new/rebuilt master cylinder. I once installed a Raybestos clutch master cylinder on a Mazda Miata and there were several warnings not to use bench bleeding.

 

[Trav]

I bench bleed all the time if its off the car but there are more ways to do a job. Putting a piece of thin wood like plywood under the brake pedal will protect the MC.

 

[Spetz]

Does the BMC have any control over brake bias or is that solely the purpose of the brake prop valve?

I am just wondering why the BMC has more than 1 outlet if the prop valve splits the fluid?

 

[Trav]

In general the master cylinder does not control brake bias although I am sure there were some dual piston MC with different size bores but they would be rare. With ABS and 4 wheel disc yours probably only a distribution block.

 

[andyd]

Usually the part will come with directions.

 

[Spetz]

Hi guys just an update on this one...

I received the BMC (second hand) and it has some brake fluid at the bottom of the canister.
Does this mean I do not need to bench bleed it as it didn't run dry?

And additionally I have the BMC with the booster as well (they are still attached).
Can I swap them in in one piece or must I still install the booster followed by the BMC?