Brake bleeding issue.

[dave64]

VX sedan.
I am not new to bleeding brakes.
Fluid change long over due on this car. Warning light would come on, if pedal pumped light would go out and stay off for days until driven hard.
New master cylinder fitted (PBR), all calipers serviced and lines checked.

Problem:
Pedal was hard while bleeder valve open (plenty far enough) during bleeding, all 4 wheels. However, brakes did bleed and seem good after test drive. But I am curious as to why pedal was hard during bleeding. Have never experienced this and a search yields no results. Seems something is not right

Thanks in advance.

 

[stick3]

warning light would tell you its low on fluid unless there a electrical fault and a hard pedal when you undo the bleed nipple it could be blocked removed it and blow or push a bit of wire through it and did you bench bleed the new master cylinder it can be done on or of the car

 

[dave64]

Thanks, but this is what I was afraid of.

Reservoir was never low and there are no electrical issues. I suspect old fluid was boiling, if not that, a master cylinder issue which has been replaced with new genuine PBR unit (maybe it is faulty, stranger things have happened).

Calipers have been serviced and bleeders where not and are not blocked. Given the issue is on all 4 wheels all 4 bleeders would have to be blocked. All bleeders have always had and still have there caps to prevent ingress of dirt and prevent blockages.

Brakes did bleed, and are working good, but i need to know why the hard pedal while bleeding.

No, I did not bleed MC. Manual I have does not say this is necessary.

f so, why will it cause a hard pedal when bleeding?????

 

[dave64]

mmmm found the following, does fit, sort of.

VALVING ISSUES
Another culprit responsible for a hard pedal could be the combination valve, and in particular the Pressure Differential Valve within that valve. This valve is there for safety reasons but can cause headaches if things aren’t working properly.The Pressure Differential Valve is designed to move should pressure drop on the front side of the valve versus the rear side and vice versa. When this happens, fluid coming from the master cylinder basically hits a wall. Because the brake fluid can’t be compressed, the pedal becomes rock hard. You still can generate moderate pressure on the “good” side of the valve therefore allowing the car to be driven in a limp home scenario. This condition is what is commonly known as and referred to as a tripped valve. If your pressure differential valve has been “tripped” it must then be re-centered by equalizing pressure on both sides of the valve. This is accomplished by getting the valve to move the proper direction and therefore putting the valve back in the center. Of course, the problem that caused the valve to “trip” must also be corrected at this time. If your vehicle has OEM disk brakes it is highly likely that it also utilizes some form of valving within the brake system. In this case, the same valve that operates the warning light on your dash – the pressure differential switch – could be the problem creating a hard brake pedal. Follow our plan above to on the right path, but as always, be cautious and employ a professional if you are unsure. Properly operating brakes are essential for safety. We are always here to get you the right equipment or additional help if you need it.

 

[losh1971]

What did you use to bleed the brakes with?

 

[greenacc]

That is a strange one. I would pull a bleed nipple right out and see if the pedal becomes softer and check nipple isn't blocked or rusted.
If that doesn't work I would open up a caliper and have a look inside then work all the way back to the master cylinder. Someone on here had a blocked brake line, I think it was Losh?

 

[losh1971]

Nah not me @greenacc i only had seized slides when i recently rebuilt my brakes.

 

[dave64]

Please read first post guys.

Calipers had been serviced = dismantled cleaned painted new seals boots lubed with bendix Ceramasil.

There are "no" blockages, all parts including "sliders" lubed.

 

[dave64]

All 4 wheels guys.

 

[dave64]

What did you use to bleed the brakes with?

Same thing I have used for 40 years.