Brake valve with prefill chamber unloading valve

A two stage brake valve for communicating fluid pressure to a brake mechanism, the brake valve having an inlet for receiving pressurized brake fluid from a pump, a reservoir port, a stepped bore and a stepped piston, the stepped bore and the stepped piston forming a high volume prefill chamber and a high pressure chamber, characterized by:

• a prefill drain valve which communicates the prefill chamber to the reservoir only when pump pressure is available at the inlet, the prefill drain valve comprising:
• a prefill drain valve bore is formed in a housing of the brake valve;
• an inlet passage communicates fluid pressure from the inlet to an inlet end of the prefill drain valve bore;
• a prefill passage communicates the prefill chamber to the prefill drain valve bore;
• a drain passage communicates the reservoir to the prefill drain valve bore;
• a valve member is slidable in the prefill drain valve bore, the valve member comprising first and second lands separated by an annular groove, an end of the first land being exposed to fluid pressure in the inlet end of the prefill drain valve bore, a valve passage extending through the valve member and communicating the groove with an end of the second land which faces away from the first land;
• and
• a spring engaging the valve member and biased to urge the valve member to a position wherein the second land blocks the prefill passage and closes communication between the prefill passage and the drain passage when pump pressure is not available at the inlet, the valve member moving against the bias of the spring to a position wherein communication is open between the prefill passage and the drain passage via the groove, the valve passage and a portion of the prefill drain valve bore when pump pressure is available at the inlet.

A brake valve assembly having left and right two stage brake valves for communicating fluid pressure to respective left and right brake mechanisms, each two stage brake valve having an inlet for receiving pressurized brake fluid from a pump, a reservoir port, a stepped bore and a stepped piston, the left stepped bore and the left stepped piston forming a left high volume prefill chamber and left high pressure chamber, the right stepped bore and the right stepped piston forming a right high volume prefill chamber and a right high pressure chamber, characterized by:

• a prefill drain valve which communicates the prefill chambers to the reservoir only when pump pressure is available at the inlet, the prefill drain valve comprising:
• a prefill drain valve bore is formed in a housing of the brake valve assembly;
• an inlet passage communicates pump pressure from the inlet to an inlet end of the prefill drain valve bore;
• a left prefill passage communicates the left prefill chamber to one side of the prefill drain valve bore;
• a right prefill passage communicates the right prefill chamber to another side of the prefill drain valve bore;
• a drain passage communicates the reservoir to the prefill drain valve bore;
• and
• a valve member is slidable in the prefill drain valve bore, the valve member comprising first and second lands separated by an annular groove, and a stem which projects from the second land, an end of the first land being exposed to fluid pressure in the inlet end of the prefill drain valve bore, a valve passage extending through the valve member and communicating the groove with an opening in an end of the stem;
• and
• a spring engaging the valve member and biased to urge the valve member to a position wherein the second land blocks both prefill passage and closes communication between the prefill passages and the drain passage when pump pressure is not available at the inlet, the valve member moving against the bias of the spring to a position wherein communication is open between the prefill passage and the drain passage via the groove, the valve passage and a portion of the prefill drain valve bore surrounding the stem when pump pressure is available at the inlet.