PV brake master cylinder
 

How do I diagnose a failing master cylinder, what are the signs?

Ok...having to pump the brake to get it to do anything....also look up at the pedals on the inside of the car, you may also see fluid on the bulkhead near the cylinder.. you may see fluid. You may also experience the pedal sinking to the floor

I don't need to pump but I get a "sponginess" not a firm solid feel. Problem is that I know that PV drum brakes are not the best but I have no other PVs here to compare. Just fitted a new wheel brake cylinder and bled the brakes. Don't get a crisp "stop" Wondered if the master cylinder seals were worn or perhaps a worn cylinder

If the brakes are spongy and you have bled them fully , non of the flexis bulge under full pedal pressure and car running..then without looking at it you could have a master cylinder issue..the pedal would in extreme circumstances sink to the floor. The danger here is that if you have a servo then this too can give you an insecure feel when applying the brakes...best and simplist trick for the servo is to pump the brake until it's hard and start the car...if the pedal does not move or it continues to travel to the floor after initial give then the servo is stuffed.

Was the pedal spongy before you changed the wheel cylinder? If not, you probably need to bleed some more.

Whenever I apply the brakes a little voice comes from behind the dash and says, "do you really want me to stop"

There's nothing much to go wrong with them - if you're having trouble bleeding try draining some fluid out and then use a pressure bleeder to push fluid back up through the system from each bleed nipple.

Arcturus;

I agree with other posters...spongyness is typically a sign of air in the brake line being compressed...bleeding (again or more effectively) is called for. Other MC failure symptoms are a bypassing seal with symptoms of a pedal which drops slowly (or faster if bypass is worse) with no apparent signs of leaks anywhere...

My final road test (in the absence of a brake force test stand) which checks for comparative braking force at each corner, is to do low-speed stops of increasing pedal force on level, slightly sanded pavement (like a parking lot, where tests can be safely undertaken), until the lockup point is reached, when this point is reached, inspect and compare the sand traces to verify similar braking at each corner...if one corner is much less, it needs attention or adjustment or other.

Cheers

"I don't need to pump but I get a "sponginess" not a firm solid feel. Problem is that I know that PV drum brakes are not the best but I have no other PVs here to compare. Just fitted a new wheel brake cylinder and bled the brakes. Don't get a crisp "stop" Wondered if the master cylinder seals were worn or perhaps a worn cylinder"


I don't know anything about the Volvo PV brakes but you may be describing two different symptoms. "Sponginess" suggests air in the system. "Don't get a crisp "stop" may very well suggest something else.


If you pump your pedal twice in quick succession, and it has more travel on the first pump than on the second, that suggests air in the system.

If you feel equally firm pedal pressure when you pump the brakes, but the car doesn't want to stop - as in brake fade associated with coming down the mountain - I'm wondering if your brake shoes are a) not working correctly because of improper installation or improper adjustment or b) that your brake shoes or drums have been contaminated by brake fluid when you replaced the wheel cylinders.

Since most braking is from the front wheels, I might jack up the front end and have someone apply the brakes as you spin the wheels to see what happens.


Assuming air in the system that despite properly sequenced bleeding is causing the sponginess, try this snake oil remedy:

Tightly block down your brake pedal with a board, stick or other device and leave it overnight. The hydraulic pressure should force tiny air bubbles out of the brake fluid and they will tend to rise to the master cylinder. Alternatively they might migrate to the wheels. Usually when I do this, my car is on jack stands and I do a quick gravity bleed at the wheels and some air is sometimes present.


I don't think it was you, but someone on these boards was asking about competition brake lining material. If your brake linings are for competition, they may not grab until they are heated up. May feel "wooden" until hot.

belt and braces called for. I will bleed again for the time being,and also fit a new master cylinder later. After all it's over fifty years old. Got a lot of expense at the moment and the PV is going to be parked up in garage 'till early Spring.