Stuck brake piston

gunna

gunna

Daily Driver
Hi all
My brakes were not very good, seemed O.K whilst tootling around town but first time I took it out for a proper bit of speed I realised how much bleeding my brakes still required.
Well tonight i decided to get the other half to do the pedals whist I shouted the UP DOWN commands, well half way through boredom must of taken over and she started pumping very hard, calmed her down and thought all would be well as no air coming out the pipes any more.
WRONG now I have absolutely no brakes at all so suspecting a stuck plunger in the master brake cylinder.
Does anyone have any idea how this can be freed off without laying upside down under the bloody dash board again and removing offending article.
It was a new cylinder and only done 100 miles since i rebuilt the whole car so really really hoping someone has a cunning plan.
 
When you say "now I have absolutely no brakes at all", do you mean the pedal won't move or it moves but it does nothing? The answer to that will help determine a better answer to your question.

First I'd verify that it is the master, and that it is a stuck piston. Have you tried cracking open a fitting at the master to see if you get any movement/fluid flow?
 
Hi Dr. Jeff

We have fluid transfer if a bleed screw is cracked open and pedal does go up and down (just) but there is absolutely no pedal pressure, pedal goes all way to the floor, that's what makes me think the plunger at the end must be stuck, if I remember the drawing in the haynes manual I believe there is a solid rod with 2 washers and a plunger with 1 washer that I believe checks fluid somehow to give the pressure. Of course I could be wrong! I will have a look at the manual later when I find it.
 
If I understand everything correctly, sounds like the master cylinder's seal gave out.
Perhaps when your assistant pushed down hard on the pedal it was enough to cause a failing seal to finally let go. Either way, it seems you will be pulling the master out of the car.
 
It does sound like a failed master cylinder, although in the course of pumping out of phase with the bleeders, a considerable amount of air could have been introduced.

I presume you are certain the reservoir never ran dry, particularly the back half as it can do so given that the two stages are separated by a dam which can allow the rear to run dry even though it looks to have fluid still in the front serving section of the reservoir.

Hopefully not the master but no surprise if it is. Sometimes pushing the seals all the way through the full stroke of the bore can snag some detritus in the MC and wear the seals so they no longer seal properly allowing the fluid to go past the seal and no longer maintain pressure.
 
I experienced a similar problem when I replaced the master cylinder and the hoses to the reservoir. My problem was an air lock between the reservoir and the master cylinder and no matter how much I bled the brakes, I couldn't get good pressure. The only difference between your scenario and mine was that I had *some* pressure, just not sufficient. I bought a vacuum pump and bled the brakes by sucking fluid through the bleeder valves, which forced all the air bubbles out. If you can pickup a cheap (or borrow) a vacuum pump, you might have success.
 
Doesn't sound promising then, it's a real bugger as only done 100 miles and was a new cylinder, I will try to get it working and post the outcome.
 
Hydraulics bleeding on our cars seems to be a combination of fluid dynamics, voodoo, and luck, with many folks trying all sorts of solutions.

Remember that there is VERY little height difference between the reservoirs and the masters and the fluid has to travel more or less three to four feet thru almost-level hoses in the scuttle to get from the reservoirs to the masters. If the hoses are not routed properly around the heater inlet in the center of the scuttle, that makes it even harder for the fluid to get to the other side of the car.

Solutions that help gravity move the fluid thru the scuttle hoses seem to be the best, whether it's very mild pressure applied to the reservoirs or vigorous vacuum applied at the slave/caliper bleeders. At all costs keep the reservoirs topped off as Karl noted, as one moment of indiscretion allows air into the hoses and now you start all over again!

I think there's science behind it, but when I did mine and suspected air in the scuttle hoses, I applied human-generated vacuum to the reservoirs to try to pull trapped air out. It was successful enough to the degree that it allowed more brake fluid flow thru the hoses so that the normal pump-and-hold bleeding process could proceed.
 
gunna said:
only done 100 miles and was a new cylinder
Click to expand...
I missed that part. Given this, unless it is a faulty/poor new master, then it is more likely to be a air issue as the others already suggested. I have good luck with applying low pressure to the reservoirs to push the air through, but any method that you have available is worth a try before removing the master again.
 
Another issue of vigorous bleeding via pedal is that the piston tends to travel further than it does during normal braking.
If the master cylinder has a few years on it, it is very possible that the area of that is not normally visited by the seal is rusty.

This results in the seal being torn/ripped/unable to seal.
 
Andy said:
Another issue of vigorous bleeding via pedal is that the piston tends to travel further than it does during normal braking.
If the master cylinder has a few years on it, it is very possible that the area of that is not normally visited by the seal is rusty.

This results in the seal being torn/ripped/unable to seal.
Click to expand...

The OP did say that the master is "new" and only has done 100 miles, but your point is still valid, as any "new" master cylinder is most likely new only in the sense that the X1/9 owner just bought it from a supplier. It's likely that the part has been sitting on shelves for years on end, and although one would hope the manufacturer/remanufacturer would have taken steps to preserve the inside finish of the cylinder, can we really take that to the bank? Thus we should not be surprised if the "new" master cylinder suffered from the same issue, rust in the bore damaging seals or preventing a good seal.
 
Well I tried back feeding fluid through the system, no visible bubbles in the reservoir and made no difference so think when I get chance I will clamp off the reservoir pipes (i have thin EPDM pipes) and crack open the end of the master cylinder to have a look and see if i can find what's going on.
 
gunna said:
Well I tried back feeding fluid through the system, no visible bubbles in the reservoir and made no difference so think when I get chance I will clamp off the reservoir pipes (i have thin EPDM pipes) and crack open the end of the master cylinder to have a look and see if i can find what's going on.
Click to expand...
In my experience, undoing the end plug on the master cylinder takes a lot of torque, easily enough to tweak the pedal box. I would take the master cylinder out of the car before trying to take it apart.

You can see the guts of my master cylinder in this post.
 
also having the passenger side (where the reservoirs are) higher could help with getting brake fluid to the Master. (filling it and going away to see if any air works its way out of the feed lines)
 
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