Front master cyl problem

I have standard front brake master cylinder, & new grimeca caliper set up, I had a look at the piston, seals etc., in the cylinder and the piston had a bit of corrosion on it so I got a kit and replaced all the parts. when I put it on and connected up it pushed oil through it no problem but the caliper pistons would hardly return leaving the piston hard against the disc. thinking there was something blocking the return I stripped down the master cylinder and reassembled it, it will not now pass fluid!. On the bench if I pour fluid in it will come out the hose connection where it's meant to, if I fit the spring with the valve and plastic piece and the primary cup, put fluid into the reservior and push it in and out with my finger I get fluid coming out the hose connection, but when I put the piston ass. in nothing comes out, the fluid in the reservior doesn't go down which suggests a blockage but all the holes are clear, I even put the old parts back in and still nothing is coming out, It's got to be something very simple but I'm banging my head off a brick wall here and missing it. Hopefully somebody can shed a light on what I'm missing.
Cheers

Old hoses will distort and act as one-way vales, allowing fluid under pressure from the master cyclinder (lever pressure) to squeeze through, but disallowing fluid return under significantly less pressure from the caliper's inner (pressure) chamber.

Get a new hose and all will be well. I suggest a high quality braided stainless one from Goodridge.

grandpaul said:

Old hoses will distort and act as one-way vales, allowing fluid under pressure from the master cyclinder (lever pressure) to squeeze through, but disallowing fluid return under significantly less pressure from the caliper's inner (pressure) chamber.

Get a new hose and all will be well. I suggest a high quality braided stainless one from Goodridge.

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Hi grandpaul , everything is new including braided hoses, the master cylinder will not even pass fluid when off the bike on the bench with no hose attached, it will let fluid through all the holes when there are no internals in the cylinder which indicates to me there are no blockages and leads me to think there is something not fitted right, I have got to have missed something simple, but can't think what.

Is the new piston exactky the same as the old one ?
Have you changed the lever or anything else?
Lastly can you post a few photos?
Cash

cap vent, proper brake fluid - come to mind although your symptoms seem more severe.

Can you describe all the parts in the order which they are fitted?

The repair kits often have an additional seal and thicker washer which should not be fitted (use only the thin curved alloy washer between the piston and primary cup).

Has the rubber 'check valve' (the first part that is fitted) actually got a tiny hole in the centre (I was once sent a repair kit and the hole in the check valve had not been punched out!)

Does the face of the new piston have four tiny holes drilled through it?

L.A.B. said:

Can you describe all the parts in the order which they are fitted?

The repair kits often have an additional seal and thicker washer which should not be fitted (use only the thin curved alloy washer between the piston and primary cup).

Has the rubber 'check valve' (the first part that is fitted) actually got a tiny hole in the centre (I was once sent a repair kit and the hole in the check valve had not been punched out!)

Does the face of the new piston have four tiny holes drilled through it?

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Order of fitment
1 Rubber check valve with plastic valve , hole open in rubber, centre hole also open in plastic piece, fitted into larger diameter end of spring
2 spring
3 plastic plug on smaller diam end of spring,
4 rubber primary cup washer
5 one curved alloy washer , raised section facing piston
6 piston
7 secondary rubber seal
8 rubber boot,


The face of the new piston has 3 holes not 4 but the old one was the same.

I got some fluid coming out a few minutes ago by using my thumb to push down the piston but when I held it in and put the lever back on all I got was a few drips.
I don't get it, there is not a lot in there to go wrong , it's got to be something simple and I'm looking past it .
Thanks for all your help guys

rbt11548 said:

The face of the new piston has 3 holes not 4 but the old one was the same.

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Yes you are right, there probably should be three holes and not four.

rbt11548 said:

I got some fluid coming out a few minutes ago by using my thumb to push down the piston but when I held it in and put the lever back on all I got was a few drips.
I don't get it, there is not a lot in there to go wrong , it's got to be something simple and I'm looking past it .
Thanks for all your help guys

Click to expand...

The business end of the masterclinder, the primary cup has to pass back and forth over the front hole in the reservoir. That hole feeds the fluid that the primary uses to pressurize the brake line after it's past it, but it also vents the line to atmospheric pressure when the primary cup moves back to resting by the force of the spring. It happens every time you squeeze the lever and let it go.
I've found it helps to take the front wheel off and use a piece of wood or something soft between the pucks to play a little tug of war between the caliper and the master cylinder. With the lever released when you spread the pucks the fluid should spout out of that first hole. For that to happen the primary cup has to be retracting all the way. It sounds by your account that it may not be. The question is why not.

This problem can be caused by fitting an upgrade lever, the fulcrum point is changed to give more leverage. This mod can prevent the piston fully returning blocking one or both the ports in the master cylinder.

Cash

cash said:

This problem can be caused by fitting an upgrade lever, the fulcrum point is changed to give more leverage. This mod can prevent the piston fully returning blocking one or both the ports in the master cylinder.

Cash

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Hi Cash, I have tried both the RGM upgrade lever and the standard one, I also thought like, bpatton suggested, that the piston is blocking the holes, but why? It was working fine before with both levers, passed it's MOT no probs with the RGM upgrade one fitted, but I found the span a bit big for my hand so I changed it back to standard, checked for operation was fine, which it was with caliper pistons returning, then went on a short run and discovered the brake seizing on, I let it cool down rode back without using front brake,and thinking it was maybe just a wee bit of debris blocking something stripped the master cyl down to clean it, put it back together and now this! I've tried the old set as well and it still does it. I've looked at the exploded diagram again and again and everything is as it should be, checked valve holes open, the valve holes are quite small and the valves sitting in the spring snugly, checked seal positions sitting correctly with the flared end of the primary cup facing into the bore, and the dished washer sitting with it's flared end towards into the bore. When the internals are out oils will pass through all the holes in the main body without a problem. I have to have missed something and it's going to be that simple I'm going to feel a right eejit for having you guys wracking you're brains. I'm going to take a few hours respite and try again.
Thanks

Rbt, Don't get the wrong idea. This is fun. It's like a Norton game show. And, the only way to have an answer is to probably having been in the same fix. You are actually very fortunate to be doing this at home. There have been cases where people have altered levers and linkages in rear brakes and had their brakes lock up when the fluid in the line expands due to heat at the cal[per.

I don't know how it's happened and I'm not going to take it apart to find out why; but after deciding to have the day off from my master cylinder problem I relented and went down to the shed to tackle it once again, There it was sitting all lonely and forlorn looking, actually that might have been me :? !, and all I did was put the internals back in, fit the original lever put some fluid in the reservoir, put my finger over the outlet hole, pulled the lever a few times and I could feel pressure building up, took my finger off, pulled the lever and out shoots brake fluid!!!!!!!!!!, so I pulled the lever a few times and each time out comes a jet of fluid! At first the fluid was coming out in different amounts, then it started to be roughly the same. So I've left it sitting in the vice and I'll check it later and make sure it's still delivering fluid, fingers crossed, and then if everything is ok I'll put it back on the bike.
Thanks everyone for all your suggestions, it looks as if now it's ok, as I said, I don't know what I did differently , hopefully it's working and one eejit now has a front brake !

rbt11548 said:

and all I did was put the internals back in, fit the original lever put some fluid in the reservoir, put my finger over the outlet hole, pulled the lever a few times and I could feel pressure building up, took my finger off, pulled the lever and out shoots brake fluid!!!!!!!!!!, so I pulled the lever a few times and each time out comes a jet of fluid!

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With your finger over the outlet, you succeeded in creating a slight vacuum downstream of the piston as it retracted which helped draw fluid into the cylinder, so your problem was due to air in the cylinder (as these hydraulic M/Cylinders make poor air pumps!)

Sometimes a similar thing can be achieved by placing the palm of your hand over the open reservoir and pressing down sharply a number of times, as that can help push the air out of the cylinder.

This probably wasn't the reason the caliper pistons were sticking on though, but see how it goes.

L.A.B said:

This probably wasn't the reason the caliper pistons were sticking on though, but see how it goes.

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No, you are right, but as I said it was probably a blockage on the return, so I had to clean the master cylinder anyway. Here's hoping everything will be ok on the bike.
Thanks to all!!!!!!!!!!!