You're describing the "monovalve". It's located between the two firewalls just to the inside of the battery on the passenger side. You may not see it because of the hoses that cover it. It has a round stainless steel top and outside this top is a flange with four screws.

Be careful removing the screws. If you drop one, be prepared to go to China to retrieve it.

When all four screws are out, the piston and diaphragm can be removed. Check for any tears in the diaphragm. It's the most common culprit. If you have a tear report back and we'll tell you where to get a replacement.

If the diaphragm is OK, then further tests of the valve are necessary to determine if the valve iteself is the problem or the CCU is the problem.