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No guarantees implied and use following info at your own risk.
From my experiences with a few 72's it seems that there is not a lot
of reserve braking power designed in the system. So when something
is not working at its optimum level the brakes are next to useless as
you describe.
As George mentions the booster is one of the areas that
can cause problems. There is a seal that lives in the booster and
straddles the shaft(rod) that goes between the booster and the master.
It stops the vacuum stored in the booster from leaking out via this shaft
and in fact if really worn out the vacuum leaks right by and is not
stored up in booster. One could prove this situation exists by temporarily
blocking the hose that goes from the manifold to the booster
(toward the engine) and noting the reduction of engine idle rpm.I've found this seal to be worn out and have bought it new from Volvo although I have not tried to buy it lately. Next , if the booster is holding
the vacuum properly then you should find the booster actually STORES
the vacuum AFTER the engine has been shut off for a period of time.
When wiggling the push in plug at the booster where the vacuum hose from manifold is attached you should hear the vacuum escaping. This will give you an idea if the vacuum is being stored. Not sure if that is the only
failure consideration for the booster.
Assuming the system is bled properly and the master and booster are
functioning properly, the next area to check is to verify that all the pistons move freely in the calipers. That is you can easily push the
pistons back into the caliper from where they reside when the pad is
removed. Clean off dirt etc from the piston before pushing into
the caliper. Don't push the pedal down with pad(s) out or the piston
will be pushed right out of the caliper. If the pistons do not move relatively freely, or show signs of leakage, rebuilt calipers are required.
**The item I found that changed a long standing poor braker into a
car that could actually lock the wheels related to the free movement
of the pads in the calipers. We all know what it is like to pull stubborn
pads out of calipers. Well when the 72 system tries to squeeze the
disc with pads that are so reluctant to move it just can't get the
job done. I found cleaning up the crud off of ALL the calipers and pads so the pads just slide in/out with absolutely NO friction will make
for a dramatic improvement. Just do what you have to do to make sure the
pads have FREE movement in their home and their future path of travel
as they wear. The locating pins should renewed and new clips used.**
As George noted I have also had hose failures both from rust in
the steel lines and from collapsed and cracked rubber hoses. Not so
obvious to find these...but if your rubber hoses have never been changed
then they are 15-20 years overdue. Replace the rubber with stainless
steel braided if available. As far as the solid steel lines are concerned
I had to replace all the lines due to internal rust and so aquired a
flare tool but couldn't figure out why my flares kept leaking until
someone mentioned that the North American 45 degree flare tool and the ISO?? flare on the 142 of 37.5 degrees are not compatible...had to buy the lines from Volvo.
The warning light works off of either the handbrake physical switch
or a pressure sensing device that compares the brake pressure between the two circuits of the dual circuit system. First with ignition switch on
put on handbrake and brake warning light should light..if not try bulb or
other connections to hand brake switch or switch itself.
A pressure sensing piston in the switch junction moves when there is a difference in pressure between the two circuits causing the
switch to activate sending a ground to the warning bulb. When you
have bled the brakes you will need to reset the switch...just unscrew it from the junction block and screw it back in. You can have a look in
the junction block while its out and see if it looks rusty. To test
the warning circuit before you reinstall the switch you can ground the wire going to the switch (ignition key must be on to test) and the
warning bulb should light...if it doesn't then the bulb or wire are suspect. Then attach the wire to the switch and touch the switch base to chassis ground and the warning light should light. When the switch is screwed back in the junction the plastic tip on the switch will be depressed so the switch does not send a ground to light until a failure occurs.
No guarantee implied and use info at your own risk.
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