Brake pedal hard as a rock, brakes locked on.

[rycymru]

Last year i replaced the brake master cylinder after it failed. The clutch master cylinder recently starting failing as well, and i replaced it. I bled the clutch system and works as it should. After bleeding the brakes since the master cylinder was removed as well, the brake pedal is now rock hard, and the car won't move because the brakes are over pressurized. Everything is put together as it should, and i don't think the brake reaction valve is bad since it was replaced last year as part of the new master cylinder. I checked the vaccuum system and no obvious damage. The system worked perfectly before replacing the clutch master cylinder.

Any idea what else it could be? Am i missing something obvious? Thank you all for any help.

Ryan.

[mgcjag]

Hi Ryan.....in the far end of the master cylinder is a small metal flap that when the pedal is operated the flap pushes on a tiny piston that operates the reaction valve, if the flap or piston sticks and does not release then tne brakes stay on.....you can release it by undoind the screws that hold the white plastic cover on the valve, take care and cover with a rag to catch the tiny pipiston, .....when i had this hapen i looked at the metal flat in the cylinder and noticed that the edges were slightly rounded on one side.....i reversed its position in the cylinder and all worked ok.....you may have to look at the tiny piston, it has a rubber o ring seal...hope this helps Steve

[PeterCrespin]

Any idea what else it could be? Am i missing something obvious? Thank you all for any help.

Ryan.

First diagnostic - hydraulics or vacuum? Method? Disconnect engine vacuum line and plug manifold port.

If brakes operate normally without binding (just harder to stop) you have a vacuum issue. If no different you have an hydraulic issue. Think of it like ignition or carbs for an engine problem, but easier to figure out. If you don't make that basic system check first, there rest will be time and money wasted unless you guess right first time by fluke.

[rycymru]

I was able to solve the problem by replacing the reaction valve at the end of the master cylinder. I already had a replacemtnt in a box of parts, so it worked out well.

Now, you really need to press the pedal to get the car to stop. I can hear a bit of hissing near the booster. Obviously, some of the rubber parts are showing their age. Next up is to see what exactly needs to be fixed/replaced to get the car up to 100%.

Thank you all for the advice.

Ryan.