Weird brake problem

[Cheap6]

It does sound like the M/C.

so whats happened most likely is that where the pistons was in the master cylinder it has worn into the alloy a little with the old pads, when you fitted the new pads the piston has moved back some in the master cylinder pack past the wear point. there may be a small ridge/lip in the master cylinder where it has let fluid past the piston allowing it to travel a lot further.

Sort of but the M/C pistons don't move during pad replacement, only the wheel cylinders do. What can happen is one of the M/C pistons moves further down the M/C bore during the process of bleeding and scratches the seal(s). It's also possible a lump of debris (product of corrosion) has been caught between a seal and the M/C bore. That might dislodge as the M/C piston returns and the load comes off the seal if the pedal is pumped.

 

[Immortality]

Sort of but the M/C pistons don't move during pad replacement, only the wheel cylinders do. What can happen is one of the M/C pistons moves further down the M/C bore during the process of bleeding and scratches the seal(s). It's also possible a lump of debris (product of corrosion) has been caught between a seal and the M/C bore. That might dislodge as the M/C piston returns and the load comes off the seal if the pedal is pumped.

yes and no. yes, when you fit new pads you push the pistons back into the calipers. you will also note that you have less pedal travel when applying the brakes with new pads so the piston in the MC is moving/working in a different spot then when you have old pads and generally have more pedal travel. either way a new MC would be good insurance.

i do agree though, when the MC has only moved in a certain range for years it tends to wear in a such, when bleeding brakes and pushing the pedal right down can lead to problems.

i have heard someone suggest that you could put a piece of wood under the pedal to prevent max travel to try and eliminate this hazard.

 

[VrWagz1]

Agree with immortality an cheap6, Definatly sounds like the master was bypassing and the piston was travelling further down the bore where it possibly sealed again. Makes perfect sensce considering the brake fail switch is at the end of the master bore. Any brake failure like this should be treated seriously which i see you are. I wouldnt drive the car until you find a reason why that happened. You dont want that happening again at the wrong time.
If ur interested in doing the vt master upgrade, i would suggest considering doing the booster too, due to the vt unit being more powerful then the vs booster.

 

[hako]

Is it possible that when the pads were installed and the calipers bolted up, one of the pads was 'cocked' and not sitting flat against the face of the piston....this may have prevented the caliper piston from forcing the pad square against the disc. You would have a solid pedal and if it was a rear pad you may not have noticed the difference in braking (with one pad u/s). after being left overnight/expansion/contraction/murphy somehow loosed the pad which then allowed the caliper piston to seat the pad correctly (and require more pedal travel for one push). Long shot eh!
But I'd certainly be a bit worried.....but I'd agree with most posters that the MC seems a good point to start at...they are cheap enough new.

Edit...about $220 on Ebay

 

[Brett_jjj]

Thanks heaps to everyone for the replies and suggestions.Im gonna either replace the M/C for now with a new VS one. Or do you blokes reckon it might be worth just fitting a rebuild kit in the old M/C ?.I will be doing the VT conversion later though still,Ive made up my mind that I will spend the cash to do it because,for one, everone recommends it,and two, Ive always felt these cars are way underbraked,Ive owned cars with rear drum brakes that stopped better!!.So Ive just done the brakes,fitted a brand new set of injectors and finally got rid of the last of the rubber bushes,Ive now got full nolothane bushes throughout the car.The ones that were replaced were the front lower inner control arm bushes.Its thrown the wheel alignment out a bit,shes squealing through roundabouts and tight turns,a bit too much toe in by the look of it,but it still feels a lot better,excellent road feel,shes booked in tomorrow for an alignment so I will be able to then truly see what shes like.Ive also fitted a new genuine holden CAS,so its cost me $140 for pads,$180 with postage for brand new "cheapie" injectors off ebay,$120 for the CAS,and $50 for the bushes and $55 for the alignment tomorrow,not too bad I think.And free installation is always the best part,me and my two young blokes did it all over saturday arvo and sunday morning..The cheapie injectors are well worth the $170 I think,it starts quicker and idles and runs heaps smoother,I guess time will tell how they go though,and whether they will last for a good while or not.New bosch injectors were around $400 a set,and I really didnt want to go reco/rebuilt either,they want $99 on ebay for cleaned reco/rebuilt bosch injectors,the EFI places around here wanted $150 to clean and rebuild the old ones...

 

[Not_An_Abba_Fan]

When you bled the brakes, did you push the pedal to the floor?

Check the brake switch on the MC, undo it and wait for a click in the MC, that will reset the piston back to where it should be. If that were the problem it shows up as the brake light being constantly on, but it won't hurt to check.

 

[tezza]

The VT M/C booster upgrade is very much worth the effort. All 3 of my VN's have now had this upgrade and I'm very happy with the brakes on all 3. None had the calipers or rotos changed so still stock in that area. I figure we are not racing anyone so if we have to stop big time once we can give the brakes a while to cool while we change undies so no need for 1,000 worth of rotors/calipers.

So normally, most people upgrade to the VT MC and booster along with the twin spot calipers and discs. If you are just upgrading the booster and master, what is the benefit of this? Just a firmer pedal due to being a dual diaphragm booster on the VT setup?

 

[Brett_jjj]

The pedal didnt quite go all the way to the floor whilst doing the bleeding as I did the rear system first, then the fronts last,so they were both never undone at the same time, there was always some pressure at the pedal when it was pushed in.But what the you guys have said about what might have happened,I agree with,the piston has gone past its usual travel area in the M/C and its caused the seals to become temporarily displaced and theyve leaked fluid past them.Then the seal has somehow reseated properly after sitting overnight and resealed itself again.Im gonna look into getting a rebuild kit for the M/C tomorrow.When I first got the car,it looked like the fluid was all slimey and mouldy,Ive never seen anything like it,but when I cleaned it all out,and changed the fluid,it was working fine,so I have never replaced it.I probably should have.Well its gonna be done now.
yeah,I tripple checked the pads again after the systems were bled,and they all looked to be sitting fine and in the right places.None were sitting crooked..Ive had that happen before and I always recheck them after the bleeding now.

 

[hako]

Brett, if you're playing with the MC and will in the near future upgrade to VT brakes, why not throw a new VT MC in now....brand spanking new they are $210 + post on Ebay. Might also need the booster ???

 

[Brett_jjj]

Brett, if you're playing with the MC and will in the near future upgrade to VT brakes, why not throw a new VT MC in now....brand spanking new they are $210 + post on Ebay. Might also need the booster ???

Yeah,I have been reading up about the conversion, trying to get as much info as possible first,and Ive just read that if you use VE calipers instead of VT to VZ calipers,you can just use the normal VS booster and M/C instead of having to change the lot over.Apparently the VE calipers have smaller pistons than the VT to VZ type,so you dont end up with the long pedal problem..It would be way better not to have to make spacers etc for the M/C,that sounds like a lot of mucking about.And I definitely dont want to end up with a long brake pedal.It this right about the VE calipers not needing the different M/C and booster?I havent read it anywhere else,so I wasnt 100% sure about it,but thats definitely the way I will go if it is right.