Volvo Maintenance Hints for 7xx/9xx - Bill Garland's Nuclear ...

it of pad drag (very intermittent, comes and goes). Should I suspect a sticky caliper piston?
[Response: Gene Stevens] Jack up the car and take the wheels off. Have an assistant push the brake pedal
down firmly while you carefully watch the piston when the pedal is released. By design, it is the soft piston
seal reforming itself to a square cross section that draws the piston back slightly. It does not take much
interference to keep that from happening. Remember, there are no return springs on a disc brake. A bad
caliper will not "walk" back and forth with pedal pressure. (FYI, it is the piston finding its position on the
pressure seal, and taking in fluid when the pads wear, that make disc brakes self-adjusting.)
The NUMBER ONE reason for piston drag is moisture getting behind the dust seal, collecting at the bottom,
and causing a slight buildup of rust on the land between the dust seal and the pressure seal. The rust
actually touches the piston and keeps it from free movement. These seals are usually damaged DURING
service (carelessness when sliding the caliper over the pads or the use of spray goo on the inner pad). That's
why the "bad" caliper symptom often appears shortly after the first rain after pad replacement.
If the pistons move nicely, rebuilding the calipers probably won't fix your problem (but it is good P.M.). I had a
similar problem with the same single piston setup on mine, and found the stainless steel guide plates and
preload springs had little dings in them from years of service, causing a grippy surface for the pads to ride
on, so they didn't slide as freely as they should. A $12 hardware kit fixed the problem.
If you DO rebuild the calipers, scrape any rust buildup you see on the thin area between the seals. If you find
signs of rust BEHIND the pressure seal, it means there was moisture in the fluid and it was not bled every
few years. Caliper bore surface is NOT critical, so scraping is allowable, but piston surface must be perfect.
Scored or pitted pistons must be replaced.
Rebuilding Calipers
Rebuild It Yourself or Buy a Rebuilt Caliper? [Tips from Editor/Chuck Jeckell] The shop I worked in insisted
on rebuilding calipers in-house. 60% or so had pitted pistons and/or bores. Caliper repair kits don't come with
pistons or sleeves, so I'd buy the rebuilt calipers from a reputable rebuilder. Consider the cost and quality of
doing it yourself: you do not have the tools or the expertise to extract rusted bleed screws, re-tap threads,
polish pistons and bores to correct dimensions or insert sleeves, and make the caliper leak-and-seizure-free.
Buy a rebuilt unit with a guarantee.
Quality Checks on Rebuilt Calipers. [Tip from Larry] Most rebuilders of Volvo parts are up to speed on the
necessary quality of the cores they rebuild. However, Girling placed a cast-dimple at the bottom of each
"half" of the front calipers. Mis-matched, incorrectly-rebuilt front calipers will have a dimple at the bottom and
a dimple at the top. The resulting internal cross-directed hydraulic pressures will cause bleeding problems
and weird failures of an otherwise sound brake system: remember a Volvo without ABS has 2 separate
hydraulic systems. Second, take metric wrenches with you to the parts store make sure the bleed screw
threads in the caliper are capable of holding the bleed screws, and while you're at it, do a visual inspection of
the threads and seals in the caliper. Occasionally a Volvo owner/mechanic will break a brakeline as it enters
the caliper. Rebuilders have been known to ruin the caliper threads when they remove the broken flare-nut
from the caliper body, so you gotta look there also. The test will come when you install them and bleed the
brakes: look for leaks and piston seizures. Often the brake pads used in rebuilt sets are at the low end of the
quality scale; you may want to replace them. In any event, make sure the new pads will fit the rebuilt
calipers: sometimes the rebuilder refaces the inner surfaces and does not leave room for the pads. Avoid
these problems by purchasing only high-quality rebuilds from a reputable retailer who will back the parts if
problems arise.
Removing Caliper and Installing a Rebuilt Caliper. The only difficulty installing a caliper is likely to come from
corroded brake pipe or line fittings. See the notes below for tips. To keep fluid from escaping the master
cylinder with a brake line open: 1) Disconnect battery negative (or pull relevant fuse) to keep brake lights off.
2)Connect a bleeder hose to the caliper (hose routed to waste container). 3) Open the bleeder, then slowly