'89 740T brake fluid bleed

The brake pedal feel on my 89' 740 Turbo has really softened in the last
month so I decided to bleed the fluid. I checked both the brake pads and
rotor to see if they needed replacement, they didn't. So along with a
patient buddy I did the tried and true old school method of pump and bleed.
Started with the caliper furthest from the brake fluid reservoir and worked
my way to the closest caliper. After the procedure, thinking the brakes
should be nice and firm again, took it out for a quick test drive. To my
surprise there was little or no difference in the feel.

Did I miss something? Any tips out there?

 

Norbert,

Without the engine running and the servo's vacuum exhausted, the brake pedal
should feel solid, firm, if it isn't, then there is air in the system, or a
leak (indicated by pedal continuing to sink slowly), to test the servo, keep
your foot on the brake pedal, start the engine, the pedal should promptly
'sink' a little under your foot, then come to rest again, but feel slightly
spongy.

All the cars I have ever driven that are equipped with servo brakes have
always had a slightly squidgy feel to the pedal, when the engine is running;
some are squidgier than others, but all have had a 'solid pedal' with a
'dead' servo.

Did you or your mate observe whether or not there was still any tiny air
bubbles exiting with the old fluid? There should be no air whatsoever in the
exiting old fluid, even tiny micro bubbles indicate a fault, and can of
course compress a lot when you press the brake pedal. If you cannot get rid
of those tiny bubbles, it can indicate a faulty master cylinder.

You are also dealing with two separate circuits at each front wheel (unless
fitted with ABS), did you bleed both?

With the 200 and 700 series volvo, I found that if parked on level ground, I
could often 'gravity bleed' the entire system (this will NOT work for ABS
equipped models, which have a special bleed sequence), just by opening the
opening the caliper nipples, a transparent exit tube allowed me to check for
air; one man job, no worries. This method, however, does not prove the
health of your master cylinder, and any sludge buildup in the system may not
be removed with the old fluid.

If all seems well with the hydraulic system, I believe, although I'm not
sure that certain types brake of pads (if not all types) can give a 'dodgy'
pedal sensation, if they have been overheated and become glazed, also, new
pads take a short while to begin to feel 'normal', but neither of these
conditions would appear to apply to you, so I'm otherwise at a loss!

Good luck, Ken

 

Did you use clean, fresh fluid from a sealed, unshaken container? Was
all the old fluid flushed out until it ran clean? On the front
callipers there are two nipples, both of which need bleeding.

If all that was done OK, check all the flexible pipes to see if they
balloon when an assistant presses the pedal.

Are there any leaks anywhere?

If everything else seems OK, suspect the master cylinder is failing.

--

Stewart Hargrave


For email, replace 'SpamOnlyToHere' with my name

 

Oops, I forgot that one Stewart, and, I should have known, I had to change
all 4 front ones once, Duh!!! My batteries must need changing

Ken