Wilsons Sandbox

Hi all, I am hoping that someone can explain to me the function of the little valve that is in the output end of the master cylinder.  It appears in the TB, TC, TD, TF and MGA (I don t know about the TA).  In the MGA the valve is found in the brake side of the master cylinder, but not in the clutch side, so it is brake specific.  It is a small conical shaped valve made of a metal cone that has holes in it and that has a rubber skirt inside.  This valve is inside the master cylinder against the end where the brake lines are attached.  The valve is held in place by the return spring in the master cylinder.  When the brake is applied, brake fluid pushes aside the rubber skirt in the valve, flows through the brake lines, and applies pressure to the wheel cylinders which move the brake shoes to apply the brakes. 

When the brakes are released the brake return springs force the wheel cylinder pistons back causing brake fluid to flow back through the brake lines to the master cylinder.  At the master cylinder, the brake fluid forces the valve open (the rubber skirt seals the metal cone and the entire valve lifts off the end of the master cylinder).  The valve opens against the force of the master cylinder return spring, and hence allows the brake fluid back into the master cylinder.  Once the brake shoes are pulled by the brake return springs back against their adjustment cams the flow of brake fluid stops, and the valve is forced closed by the master cylinder return spring.  The end result is a slight positive pressure in the brake lines and wheel cylinders.  This positive pressure results from the pressure of the master cylinder return spring holding the valve in place. 

I can think of several reasons to have the valve:

[*] Having the brake lines and wheel cylinders at a positive pressure would help prevent air or moisture from squeezing by leaks or seals and contaminating the brake fluid.  The brakes may come on a little bit faster.  Although, I suspect this would have minuscule effect.  If there were a leak in the brake system and fluid was lost under braking the valve would help prevent the development of low pressure in the brake system as fluid is sucked from the reservoir by the master cylinder when the brakes are released. 

Here is the catch.  Modern master cylinders have no such valve.  We have run master cylinders with and without the valve and have noticed no difference in operation of the brakes. 

I freely admit that the engineers who designed the brake system in our little cars are a lot smarter than I am.  They went to a lot of trouble to put this valve in the master cylinder.  I just can t figure out why. 

Can anybody explain the function of the valve that I am missing?  Thanks for the help. My best, Roger in Poestenkill

Hi Roger, The main function is to prevent brake fluid being drawn back into the master cylinder when bleeding the brakes. Brake fluid expelled through the bleed nipples in the wheel cylinders would be drawn back in (including any air bubbles) if it were not for the check valve, when the brake pedal is released. An additional benefit is the slight positive pressure in the brake lines which helps the sealing cups in the wheel cylinders splay out and maintain an affective continuous seal. A more detailed account can be found in John James' Totally T-Type 2. June 2016, issue 26. http://ttypes.org/ttt2/tabc-brakes I suspect that check valves are not needed in modern vehicles due to the use of disk brakes and different bleeding techniques. Hope this helps, Eric. _______________________________________ From: rgmorse@morse-associates.com I am hoping that someone can explain to me the function of the little valve that is in the output end of the master cylinder. It appears in the TB, TC, TD, TF and MGA (I don t know about the TA). In the MGA the valve is found in the brake side of the master cylinder, but not in the clutch side, so it is brake specific. It is a small conical shaped valve made of a metal cone that has holes in it and that has a rubber skirt inside. This valve is inside the master cylinder against the end where the brake lines are attached. The valve is held in place by the return spring in the master cylinder. When the brake is applied, brake fluid pushes aside the rubber skirt in the valve, flows through the brake lines, and applies pressure to the wheel cylinders which move the brake shoes to apply the brakes. When the brakes are released the brake return springs force the wheel cylinder pistons back causing brake fluid to flow back through the brake lines to the master cylinder. At the master cylinder, the brake fluid forces the valve open (the rubber skirt seals the metal cone and the entire valve lifts off the end of the master cylinder). The valve opens against the force of the master cylinder return spring, and hence allows the brake fluid back into the master cylinder. Once the brake shoes are pulled by the brake return springs back against their adjustment cams the flow of brake fluid stops, and the valve is forced closed by the master cylinder return spring. The end result is a slight positive pressure in the brake lines and wheel cylinders. This positive pressure results from the pressure of the master cylinder return spring holding the valve in place. I can think of several reasons to have the valve: * Having the brake lines and wheel cylinders at a positive pressure would help prevent air or moisture from squeezing by leaks or seals and contaminating the brake fluid. * The brakes may come on a little bit faster. Although, I suspect this would have minuscule effect. * If there were a leak in the brake system and fluid was lost under braking the valve would help prevent the development of low pressure in the brake system as fluid is sucked from the reservoir by the master cylinder when the brakes are released. Here is the catch. Modern master cylinders have no such valve. We have run master cylinders with and without the valve and have noticed no difference in operation of the brakes. I freely admit that the engineers who designed the brake system in our little cars are a lot smarter than I am. They went to a lot of trouble to put this valve in the master cylinder. I just can t figure out why. Can anybody explain the function of the valve that I am missing? Thanks for the help. My best, Roger in Poestenkill