Brake Master Cylinder and sticking front brakes

[Andy Mathers]

Hi there,
I need some advice please on sticking brakes.
I've just changed the brake master cylinder on my manual car, re-assembled, bled the brakes and took the car for a small spin out. Everything seemed fine but for some small reason I stopped the car short of it's usual spot in the garage whilst I cleaned the floor from spanners and bits of clutter. Instead of starting the engine I thought I'd push the car in the metre or so to fit in the garage and noticed that the car didn't easily roll as it should have done. Instead I've found that the front brakes seem to bind on. I've undone one bleed screw on the front to free off some pressure the the other front wheel, and each wheel then rotated in turn easily enough. After application of the brakes again in the garage the front wheels seem to be binding again. I've spent far too long with the car in the garage and I'm missing out on the sunshine !!! My colleague advises it might be the piston in the m/c sticking - will I have to spend another 4 hours removing and refitting the m/c and servo for another possibly dubious part ? I should add that the system has been filled up with DOT4 brake fluid.
Can anyone offer some advice please ??
Thanks in anticipation

[KOY 23]

Are the brake flexi hoses new? They have been known to act as one way valves.

[ChasR]

There is a possibility that a seal was incorrectly fitted in the MC assembly (or did you buy new in the end?), but I saw 2 year old flexi hoses on my Stag do exactly what KOY has said.

[martin]

Hi Andy, before condemning the brakes, stop facing downhill, reverse a short distance, brake then release, note what happens at the moment you stop, does she roll forward straight away, or is there a pause before moving, I don't think there's anything wrong, but if there is a pause, I'd go along with John. Martin.

[jakesmig]

Is there any slack in the pedal? Could be the master cylinder piston is not being pulled back far enough and the hole up into the resevoir is not exposed. it is the exposure of this hole on release of the brake which lets all the system presure disipate.
John

[flying farmer]

Is there any slack in the pedal? Could be the master cylinder piston is not being pulled back far enough and the hole up into the resevoir is not exposed. it is the exposure of this hole on release of the brake which lets all the system presure disipate.
John

I had this problem on my PI saloon years ago, and what puzzled me was that it just started happening with no components changed. The servo push rod was theoretically adjustable, but seized solid in practice. In the end I resorted to a pair of thin washers between the servo and master cylinder to give it enough clearance
Neil

[Andy Mathers]

Thanks for all your replies chaps - very helpful.
As a complete novice on mechanical stuff I followed the instructions in the ROM meticulously so last night I was agonising over what I'd done wrong and how rubbish I must be that I couldn't even get this right. In other words - total loss of confidence in trying to do these 'simple' jobs.
After leaving the car overnight (together with a feeling of much gloom and despondency) my colleague came around to verify my fears. So, I jacked up the front of the car and thought smugly their's no way that the front wheels will rotate cos I'd applied the brakes yesterday it has to be some deep seated problem within the bowels of the braking system. To my surprise both wheels spun cleanly with no resistence. We tested again by heavily applying the brakes and releasing....they still spun easily. Several times we tried this and each with the same outcome. The thought process is that as the master cylinder is new we may have put too much pressure for the piston in the master cylinder to release during the early bleeding sessions and by leaving overnight with plenty of fluid in the system it gradually released itself.
All being well I'm at Ripon Show tomorrow so hopefully I'll be there with the same amount of thinning hair (and a fairly straight Stag) that I started out with !!!!
Once again many thanks

[Barry]

Hi Andy,
Just a thought, but where did you get the new master cyl?
I ask this beacuse if it was a generic cyl, it may well be designed for drum brakes.

On sp;it systems such as the Stag with rear drums and front discs there is a valve in the cyl that can cause this.
Drum brake wheel cyl's will leak if there is not a small amout of pressure retained in the brake linr at all times. There is a valve in the master cyl called the "residual line pressure valve". this valve is designed to retain a small amount of pressure in the rear brake lines in order to expand the rubber cups in the cyle to prevent leakage. Disc brake calipers do not need this. thus, if the master cyl is designed for a 4 wheel drum brake system, there will be a valve in the font brake system, this will apply pressure to the calipers and hold the brakes on slightly.
the valve is usually located where the brake pipe connects to the master cyl. genrally, the valve is very easy to remove, although, if your cyl has one, just take it back and get the right one.
regars, Robin

[Andy Mathers]

Thanks for all your replies chaps - very helpful.
As a complete novice on mechanical stuff I followed the instructions in the ROM meticulously so last night I was agonising over what I'd done wrong and how rubbish I must be that I couldn't even get this right. In other words - total loss of confidence in trying to do these 'simple' jobs.
After leaving the car overnight (together with a feeling of much gloom and despondency) my colleague came around to verify my fears. So, I jacked up the front of the car and thought smugly their's no way that the front wheels will rotate cos I'd applied the brakes yesterday it has to be some deep seated problem within the bowels of the braking system. To my surprise both wheels spun cleanly with no resistence. We tested again by heavily applying the brakes and releasing....they still spun easily. Several times we tried this and each with the same outcome. The thought process is that as the master cylinder is new we may have put too much pressure for the piston in the master cylinder to release during the early bleeding sessions and by leaving overnight with plenty of fluid in the system it gradually released itself.
All being well I'm at Ripon Show tomorrow so hopefully I'll be there with the same amount of thinning hair (and a fairly straight Stag) that I started out with !!!!
Once again many thanks