140 Brake failure


Currently battling a braking issue on my 164

Symptoms after replacing the proportioning valve that has the failure sensor and bleeding are the pedal randomly going almost to the floor and loosing most of braking capacity. it happens unexpectedly which leads me to believe it may be the master cylinder.

i also am unable to lock the wheels up upon stamping on the pedal. prior to replacing the valve this was also the case however never had the pedal sink. is it possible that i simply haven't bled all the air out of the lines. the brake failure light has come on after this issue arose but not immediately



  • Air is a high possibility

    Before doing any more work; I would have someone bleed the brakes for you. The number of brakes I have fixed with just bleeding the air out of them is plenty

  • Angus242164Angus (@Angus242164) Western Victoria

    If there is air in the system, the pedal will be low but consistent each press.

    Intermittent low pedal is caused by a lack of pressure in one of the two fluid circuits, ie: master cylinder has an internal issue.

    The warning light switch is earthed by a piston fitted between the two circuits, such that it sits in the centre of it's housing when pressure is equal in both circuits. If pressure is low on one circuit, the piston is pushed towards that circuit and earths against the switch.

    There is no proportioning valve as such on a 164, just the master cylinder, warning switch housing, and a pair of what I call delay valves under the rear floor, which ensure that pressure goes to the front brakes slightly before the rears, to prevent the rear wheels locking up as easily under hard braking.

  • ab1Al (@ab1) Melbourne

    ive installed a second hand master cylinder and it has eliminated the pedal sinking randomly. however i am still unable to lock up the wheels.

    is it possible theres an issue with the booster? i had a quick look inside and seemed as though there was liquid possibly brake fluid in there

  • i had a quick look inside and seemed as though there was liquid possibly brake fluid in there

    That's a sure sign that the master cylinder has failed catastrophically. Was the fluid in the booster before or after you changed the MC? If before, you may have fixed the problem; if after, the replacement MC is a problem.

  • Angus242164Angus (@Angus242164) Western Victoria

    Yep fluid leaking from the rear seal of the master cylinder will fill the booster. Standard test is to stick a cable tie in through the vacuum port and see if it comes out wet. Brake fluid in the booster will eventually damage the rubber diaphragms and seals inside.

    There are a few tests you can do on the booster.

    If you hear hissing from the pedal area at any point in the pedal travel, is has a vac leak.

    Unplug the vac hose with the car off, if the booster is holding vacuum like it should, air will rush in when it's removed.

    With the hose unplugged and the car running, you should have good suction on the end of the hose.

    With the car off, press the pedal, the first two or so presses should be a bit soft with longer travel, after that the travel should be shorter and harder, as the vacuum held in the booster is expended.

    You can try driving it with the vacuum disconnected, and see if the brakes are worse than they already are. Be very careful obviously, they may in fact be significantly worse than they already are, if the booster is working.

    You may have some seized caliper pistons, blocked brake hoses or other faults causing a lack of force being applied to the pads.

  • Angus242164Angus (@Angus242164) Western Victoria

    Even if all those tests on the booster come up OK, you can still have a fault in the booster valve body that causes reduced boost, but doesn't show any other symptoms by which to verify the fault. This happened to my 760T a number of years ago, hard pedal and limited boost (therefore longer stopping distance) but only at a particular point in the pedal travel, sometimes you could actually "push past it". A replacement booster fixed it.

  • ab1Al (@ab1) Melbourne

    fluid was in there before master change. will have to clean it out before it eats the diaphragm.

    i shall perform those tests on a day off. if the booster if not connected shouldn't i be able to lock the wheels up with "extra" force provided callipers/lines are in working order.

  • Angus242164Angus (@Angus242164) Western Victoria

    Without the help of the booster it can be really hard work to get the wheels to lock up by human force alone, but generally it should still be possible if you really jump on the pedal.

  • ab1Al (@ab1) Melbourne

    strangely enough while i was testing the brakes the pressure returned and i can now fully lock up the brakes with excellent pressure. very strange that it decided to return by itself.

  • ab1Al (@ab1) Melbourne

    Pulled the booster and there was quite a bit of fluid in it. Drained it and cleaned it out with brake cleaner. interestingly enough i can hear a hissing when stepping on the brake. sounds like its from the booster area but booster holds vac as when i pulled vac line there was a loud hiss. but brakes are able to lock up and pull up strongly now. i will try driving without the booster connected to see if i notice much of a difference.

  • Angus242164Angus (@Angus242164) Western Victoria

    Hissing from the booster is not a good sign, is it coming from the area where the pushrod from the crossbar goes into it? If so it sounds like there's a leak in the valve assembly when the pedal is pushed.

    In this case, it will still hold vacuum when the pedal isn't being pushed.

    I may have a spare booster that I can sell you.

  • ab1Al (@ab1) Melbourne

    Thats correct. I test with the booster disconnected and there is definitely vacuum pressure when connected whether or not it as much as should be is another question but have no issues locking the brakes. However I noticed when out of gear and lock them up the rears will lock up first. Am I incorrect to think there's a valve that's meant to stop this?

    Is the hissing noise possibly just normal function of the booster or definitely indicative of a leak ?

  • carnut222Greg S (@carnut222) Daylesford VIC

    Does the hissing occur continuously while you have your foot pressed on the brake pedal, or does it just let out a “whoosh” when you push the pedal? I think some boosters make that sort of SHHH noise when you push the pedal, but once the pedal is down it shouldn’t continue making the noise. Just my experience with various cars...so maybe the booster isn’t bad?

  • Angus242164Angus (@Angus242164) Western Victoria

    There are valves in the lines to the rear brakes that delay pressure to the rear calipers, to allow the front calipers to start working before the rears come on, but in my experience the rears will sometimes still lock up under very hard braking on RWD Volvo's when everything is working as it should.

  • ab1Al (@ab1) Melbourne

    Only occurs when pushing the pedal. No noise when pedal pressure is constant

    Interesting I suppose it's probably normal functioning. I guess since I've been focussing on the brakes of recent I'm nitpicking but it's a part of the car that I want to be working at 100% especially given its a daily

  • FYI, 140s are supposed to have a 60:40 brake bias to the front. The 240 was changed to a 70:30 bias to the front.

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