Vespa GTV125 - Lazy Rear Brake Piston

Discussion in 'The Workshop' started by Dionysus, Sep 19, 2020.

  1. Dionysus

    Dionysus New Member

    Messages:
    2
    Hi everyone,

    I'm having a bit of a problem with my rear brake caliper and am hoping you may be able to give me some idea of the cause of the problem.

    The background is this:

    I bought this bike a year ago. The rear brakes have always squealed. I had it serviced after a couple of months and specifically asked them to look at the rear brakes and replace the pads, if necessary. They tole me they were fine. There's no need to go into too much detail, but I have reasonable reason to doubt their assessment: they broke a number of things on my bike which necessitated a transmission and top end engine rebuild. There's a number of things they were supposed to do for the service which also clearly weren't done, so I had good reason to want to take a look at the pads myself.

    I had my rear tyre replace today, so, whilst the wheel was off, I decided to take a look at the pads. When removing the calipers by removing B and C, I accidentally loosened A and caused about a teaspoons' worth of brake fluid to leak from the seal down the centre of the calipers (line D). (See pic: https://ibb.co/TRpdNyn ) I quickly realised and tightened it back up and carried on with the job, aware that I may have to bleed the brake line now.

    Anyway, the pads were fine and I went to reassemble everything. When I put the calipers back on, I couldn't turn the brake disc/hub. After a closer look, I could see that the left-side piston was fully engaged, whilst the right one had a good 3mm gap. So, I took it off again and had a look at the pistons' movement without the pads. The on every pull of the brake lever, the left piston would move about 4mm and drop back about 2mm. Whereas the right piston would move maybe 1mm and go back to its original starting position. After several times, the left piston would go all the way to the centre and the right on would be right back in its original position.

    To see if the right piston was seized, I held the left piston in place and pumped the brake. Sure enough, the right piston started behaving exactly like the left one had before: moving about 4mm per pump and then receding 2mm after coming of the brake lever. The resistance on the brake lever was also softer (which is significant, I think). In other words: it doesn't appear to be that the right piston is seized: it does work; it just needs the left piston to be prevented from moving for it to have enough pressure.

    I'm guessing that the obvious explanation is that I've let some air into the caliper when I loosened A. I'm not sure where that leaves me, though. I've tried two things:

    1) I bled the brake of a teaspoon or 2's worth of fluid. There was no air or bubbles. It all seemed fine.

    2) I held the left piston in and pumped the brakes to let the right piston get 'a head start' to see if that helped balance it out. Long story short: it didn't.

    Essentially, I'm unsure where this leaves me. Would doing a full brake bleed stand a chance at sorting it? Or is the caliper knackered because I loosened the seal? Or, is there some other explanation or was of resolving it?
     
  2. MARSH

    MARSH Whooooo!

    Messages:
    3,200
    Rides:
    Piaggio Typhoon
    You could strip/ clean caliper as dirt can get beneath seals and slows action of caliper pistons, not a big job but if your unsure leave it to someone who knows ! A little bit of fluid wouldn't cause a big problem. I'd sort it if you were in Leicestershire.
     
  3. Dionysus

    Dionysus New Member

    Messages:
    2
    Hi MARSH,

    Thanks for the suggestion. I've had a go a cleaning the piston by pushing the right one out and cleaning the surface, but I'm not too confident about striping the caliper to clean it. The Haynes for the bike says specifically not to do that and that replacement calipers can only be bought whole. So it's definitely not something I want to tackle myself. Would this mean that a mechanic would also not want to do it, or would it still be advisable to do it? A new caliper costs about £80-90 plus £10 for new brake fluid. Do you think a mechanic would be able to do it for less, or would it be better to go straight for a new caliper? Or would there be any other things I could try first? Like having another go at cleaning it without taking it apart? Or having a go at a full brake bleed? Obviously, I know you don't know the definitive answers to these questions - it would be nice to hear an opinion on it, nonetheless.

    Thanks.
     
  4. MARSH

    MARSH Whooooo!

    Messages:
    3,200
    Rides:
    Piaggio Typhoon
    The brake fluid is hydrascopic ? it absorbs moisture, this can and does build up crud under the thin seal in caliper thus forcing seal to hold piston tight stopping the piston to move in/out easy, I'm not saying it is the problem but worth checking. If it was mine I would strip caliper in half remove pistons to clean, any half descent mechanic should have no problems at all ! DO NOT PLAY WITH BRAKES if your not 100% confident, I can't work my P.C or mobile phone propery, I get the kids to sort it !
     
  5. Piagfly

    Piagfly New Member

    Messages:
    6
    I had problem with my stepson brakes when I changed the master cylinder turned out it was air trapped in it I thought the brakes were bled good but piston would move like yours pump the brake a bit then undo the banjo slightly with pressure on brake and air released from there and then perfect brakes after, hopefully this is your problem as well
     
  6. Stevep

    Stevep Well-Known Member

    Messages:
    5,895
    9 times out of 10 it's the dust seal that causes the problem, but that job does entail a caliper split unfortunately.
    As Marsh says, if you are not 100% confident then find a decent spanner to do it for you.
     

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