Is it necessary to bench bleed master cylinder?

[Greg S.]

If you use a pressure bleeder to bleed brakes is it necessary to bench bleed master cylinder.? Master cylinder is fresh rebuild with brass sleeve by White Post Restorations. It is dry now. Calipers and lines are also new.

[Donald O.]

Re: Is it necessary to bench bleed master cylinder?

If you use a pressure bleeder to bleed brakes is it necessary to bench bleed master cylinder.? Master cylinder is fresh rebuild with brass sleeve by White Post Restorations. It is dry now. Calipers and lines are also new.

I recommend that it be bled. This can be accomplished after installation, but on the bench is easier. I have a pressure bleeder and use it yearly to flush the DOT3 fluid.

I thinks it takes me longer to jack the car up and remove the tires/wheels than it does to do the bleeding.

DonO

[Steve G.]

Re: Is it necessary to bench bleed master cylinder?

It is necessary to bench bleed a master cyl when the installed mc is not level (horizontal) and there is an area of the bore that would be above the highest line connection. You will have a pocket of air there that is impossible to get out.

Steve g

[Frank D.]

Re: Is it necessary to bench bleed master cylinder?

I've never bench bled a master cylinder and; I've never had a problem by not doing so...

[Jim L.]

Re: Is it necessary to bench bleed master cylinder?

Like Frank, I've never bench bled a master cylinder either. Ignorance is bliss, I suppose. I probably got away with not bench bleeding the MC because I didn't know I had to.

Jim

[Steve G.]

Re: Is it necessary to bench bleed master cylinder?

It's only the cars that had the nose of the MC angled up with the lines off the side that ever required bench bleeding. If you never installed one of those you would never have had a problem. I don't know that any of these Corvettes qualified. And if you did leave some air behind in the end of the mc you would still have pedal, it just wouldn't be quite as firm or high as it wold be with all the air out.

Steve

[Jim L.]

Re: Is it necessary to bench bleed master cylinder?

I don't understand why the attitude of the MC matters and my experience has been that it doesn't.

Here is the angled MC on my '63. Last time I rebuilt the brakes on this car I was blissfully ignorant of the necessity of bench bleeding the MC. So, I didn't do it. And it hasn't mattered. The car has a firm pedal and stops readily.

[Jim D.]

Re: Is it necessary to bench bleed master cylinder?

I wonder if it has something to do with a single circuit M/C with the outlet at the end of the piston vs. a dual circuit M/C with side outlets. I've never had to bench bleed a single circuit M/C but found that with dual circuit ones, it's almost impossible to get a firm pedal without doing it.

[Mark E.]

Re: Is it necessary to bench bleed master cylinder?

It's only the cars that had the nose of the MC angled up with the lines off the side that ever required bench bleeding. If you never installed one of those you would never have had a problem. I don't know that any of these Corvettes qualified. And if you did leave some air behind in the end of the mc you would still have pedal, it just wouldn't be quite as firm or high as it wold be with all the air out.Steve

I didn't know that bench bleeding helps ensure the MC is completely cleared of trapped air, as Steve suggests. That possibility alone justifies the practice in my mind. But I bench bleed because it makes bleeding the complete system faster and easier. The time saved with the system bleed more than offsets the time to bench bleed the MC.

[Steve G.]

Re: Is it necessary to bench bleed master cylinder?

As I said in my earlier post, it's only where the line comes off the side and is not at the very end of the bore. With your cyl tipped up the line passage is still the highest point. Can't trap air,


But in this case

SAM_1518.jpg

if the mc is tilted up there is a space in the end of the bore that would be higher than the line coming off (circled in red). Air will trap there. This is often the case with dual circuit cyls.

Steve

[Mark E.]

Re: Is it necessary to bench bleed master cylinder?

Here's an interesting article I just found- http://www.superchevy.com/how-to/brakes/0509sc-bench/

An excerpt consistent with Steve's post: "If the master cylinder is not bled, it will take you at least twice as long to bleed the system, and then there's no guarantee that you'll have removed all the air from the system... Be sure to place the master cylinder level in a vice and secure it properly. If the cylinder isn't level, not all the air will be able to be removed from the master cylinder."

[William F.]

Re: Is it necessary to bench bleed master cylinder?

Draw a bit of fluid out of master so it won't run out when you do next thing.Park car with nose down or jack up rear. Remove cap from master cylinder. Run a fine wire through 2 holes in inside bottom of mc. (if dual system master) to make sure they're open. GENTLY pump brakes and you or assistant will see any air that's in mc bubble to top from holes in bottom of mc.. Park car level and replenish fluid to proper level. This method was told to me by Corvette Stainless Steel Brakes people and it seemed to work.

[George W.]

Re: Is it necessary to bench bleed master cylinder?

I A WORD YES...Mark has it correct in posts 10 and 11

[Jim T.]

Re: Is it necessary to bench bleed master cylinder?

Greg, My 1970 and 1971 GM service manuals both mention to place the master cylinder in a vice with the nose down to bleed and then the rear of mater cylinder down to bled. Last time I did this on my 1970 I did it this way. I used a kit that screwed into the master cylinder with plastic lines to run into the master cylinder. I left them in place and installed the master cylinder. After the master cylinder was installed I removed the plug from my brake line and them removed one of the plastic lines and connected to the master cylinder and lightly screwed in the brake line and let the brake fluid from the master cylinder leak into the brake line. Satisfied that the brake line was filled with brake fluid I tightened the connection and did the same for the other brake line. Brake pedal was great and I never bled brake fluid from the calipers. This was done about 4 years ago and still great braking.

[Mark E.]

Re: Is it necessary to bench bleed master cylinder?

This was done about 4 years ago and still great braking.

...and don't forget to flush with fresh brake fluid every two years or so. That prevents moisture build up and component damage.