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MG TD TF 1500 - Bleeding Brakes

Hi,
I am fixing a few things on the bake lines of my '53 TD. The whole system was replaced/rebuilt about 20 years ago with DOT 5 silicone brake fluid.

My question is: has anyone tried to use a syringe to pull a vacuum on the bleeder port at each wheel cylinder to bleed the system? I've seen systems from Moss and others where you can generate a positive pressure at the cap of the master cylinder so I thought pulling a vacuum might do the same thing.

Thanks for you thoughts.

Jon Morgan
Jonathan Morgan

In theory it should work; in practice I doubt the syringe would draw enough vacuum, unless it is a plenty-big vet syringe. Better to get a dedicated Mityvac system.

Tom Lange
MGT Repair
t lange

I was looking for a cutout picture of the master cylinder but could not find one online.

Basically I don't think this will work because of how the master cylinder works. Each time you press the brake pedal a small amount of fluid is moved from the reservoir to the brake pipes to increase the pressure. When you release the brakes this fluid is returned.

When you bleed the brakes the fluid is not returned and you have to pump it multiple times to push the fluid (and any air) out of the line, hopefully making sure you don't use all of the fluid in the reservoir and injecting air into the lines. This is why you always bleed from the closest to the farthest wheel cylinder.

If you tried to suck the fluid from the wheel cylinders at best you would only drain that smaller pump but I doubt you would even get that far. The master cylinder pump is going to prohibit you from access to the large reservoir which is what you want access to.
Christopher Couper

Chris. Closest to the farthest? Donít you mean farthest to the closest?
W A Chasser

Christopher,
the master cylinder seal collapses to allow new fluid into the system. This happens whether you manually pump,pressurise or vacuum the system to bleed.
With the vacuum system you can get kidded by air leaking down the bleed screw thread, so a bit of grease will prevent this.
I and a few friends have fitted a high level remote reservoir and now can bleed without worrying about running down on fluid.
Ray TF 2884
Ray Lee

Bill: For most cars it is the farthist first but I guess in the TD/TF it really makes no difference.

Ray: Good to hear the master cylinder can be overcome by pressure from the wheel cylinders.
Christopher Couper

Christopher,
There is a static pressure between the wheel cylinder and the master cylinder top hat foot valve (3 to 5 PSI). This is generated by the return springs retracting the pistons. The fluid is returned to the master cylinder via the recuperation hole until the static pressure is reached.The rubber cup in the foot valve opens to allow fluid into the system and the whole foot valve is held on its seat by the piston return spring and is lifted off by the pressure to return the fluid.
So the master cylinder at rest is overcome by the wheel cylinder, it has to or the brake system will not work.
the static pressure also keeps the seals pressed to the cyl walls.
Ray TF 2884
Ray Lee

Hi Jon,
Using vacuum to draw brake fluid through the system could end up drawing air past the wheel cylinder rubber seals. As Ray has pointed out the static pressure in the system keeps the seals in contact with the cylinder walls; remove this static pressure and apply a vacuum will collapse the seals, drawing in air.
Eric
E A Worpe

Eric,
No problem there, the N/R valve in the foot valve will open as soon as the slightest vacuum is applied and prevent seal collapse.Vacuum bleeders are the easiest way of single handed bleeding.
Ray TF 2884
Ray Lee

I tried using one of those little squeeze handle one man bleeders and it did not work at all. It kept drawing air from somewhere so I went back to my wife pushing the pedal and me on the bleeder and brakes bleed easy.
Bill Brown TD-24560
Bill Brown

Thanks for all of your thoughts. This is the type of well reasoned information I was hoping to get.

That being said, I figure I'll use the tried-and-true method of pressure bleeding with my wife pushing on the brake pedal.

While I greatly appreciate Ray's wonderfully thorough description of the workings of the brake system, I am concerned with Eric's comment about air getting past the wheel cylinders.

Thanks again!

Jon Morgan
Jonathan Morgan

To effectively bleed all types of hydraulic brakes the pistons must be pushed in fully and held there whilst the bleeding is carried out. You can do this by adjusting the shoes up tight or clamping the pistons with a G clamp. If you don't do this there will be a bubble of air behind the pistons and as you bleed the fluid will compress this but will not purge it out. The fluid will actually flow from the hose inlet and then out of the nipple. Don't do this and it is not or miss whether you get a decent pedal.

Jan T
J Targosz

If you suspect the air is in the cyl you can get it out.
Locate which wheel you think is at fault by using hose clamps.Fully slack off one adjuster and get somebody to depress and hold the pedal down.This will push the piston right back and introduce more fluid. Smartly open the bleed screw the resulting velocity should purge any air.
re-adjust the brake and check.
This has always worked for me.
Ray TF 2884
Ray Lee

This thread was discussed between 07/06/2020 and 11/06/2020

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