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Please help advice on a brake fault gtm coupe 
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GTM on order

Joined: Tue Nov 28, 2017 12:04 am
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Post Please help advice on a brake fault gtm coupe
Any advice on the follow would be appreciated.

My gtm has metro disc brakes on the front, and drums on the rear with a servo and has had all new slaves and new master, after bleeding in order the pedal will go to the floor and when pressed agin hold well, it does not leak and holds overnight with the pedal being kept down. However as soon as the pedal is released a small amount of fluid pushes back into the master and the pedal needs to be pumped once.

I have bled under pressure, and vacume bled (no bubbles and all lines bleeding well)
I have adjusted the drums
I notice that the once pumped and then pedal released the brake lights stay on for a second as the fluid pushes back to master.
Checked shoes fitted correctly springs etc
Cars level and the master is a single line clear plastic type

I've not run the car yet but it is ready so could this be the servo? Or am I unlucky and have a new but duff master.

I have had a mobile mechanic out (nice chap after an hour he was stumped too and did not charge me but drew the setup to get advice)

This is my last huddle apart from cosmetics.

Thank you in advance.

Wed Mar 21, 2018 11:31 pm
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Joined: Fri Oct 12, 2007 9:38 am
Posts: 740
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Post Re: Please help advice on a brake fault gtm coupe
Oooh, chasing faults like this will age you....

Here's what I would try.

Is the master at the correct angle? It may have a bubble stuck in there. Tap it a lot with a spanner while bleeding. Bleeding on a warmer day should help too.
Shuttle valve? If you've got one. Get rid.
Tiny air leak somewhere that won't let fluid out but air in. Crack up all connections. Check seal between reservoir and master.
Reverse bleed it.
Are you bleeding it with engine running? It's a very good idea when you have a servo, but also the vibration from the engine helps with shaking any bubbles around and getting them moving.

And the daftest of daft, but I have seen it done with Metro calipers - have you fitted them the right way up - on the correct side? Bleed nipples should be at the top, obviously.

Edit: now that I've read your original post a little more closely.

Try bleeding the master at the master. Undo the pipe that comes out of the top of it a bleed a little fluid out while holding a rag around the joint.

Other than that it sounds like either a lazy master or a restriction in the lines somewhere. Do the brakes stay on after the pedal is released like the brake lights do?

The other thing I'd question is a single line master on metro calipers - single line might struggle to move enough fluid.

GTM Libra, GTM Coupe, Siva Moonbug, GMC Safari And DeTomaso Pantera.

Thu Mar 22, 2018 10:58 am
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Joined: Tue Feb 23, 2016 11:14 pm
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Post Re: Please help advice on a brake fault gtm coupe
Brakes are simple hydraulics. if the peddle drops or goes soft you have air in the system. If you bleed the system and there is no air in the lines. Then the pedal goes back to being soft you have drawn air into the system. If you are drawing air into the system it can only be coming from the cylinder seals. If it was a bad join it would leak under pressure of the brake pedal being pressed, which should be obvious.
So lets check the system starting with the easy stuff.
1. check the fluid level is at the mark. If the fluid is very low you can draw air into the system through the reservoir.
2. pump the pedal till it is hard, hold the pedal down as though you are breaking, then start the engine. The pedal should slowly go down about 25mm. If it does your servo is fine.
3. follow all the brake lines and check for leaks, it helps if you have someone to keep pumping the pedal. A leak should show up fairly quickly.
4. Now we get to the more time consuming. Check each of the pistons on the brake system for leaks. You will need to pull the rubber dust covers back as you can have a leak which can be contained behind the cover and you wont find it without a proper look. For some reason the cylinders on brake drums are very prone to this.
5. check the master cylinder for the same leak behind the dust rubber. This is the one I would be suspect of from your description.

If you find any faults then its a mater of sorting them and checking for improvement in the pedal.
If you don't find a fault you need to start a more thorough check of the system for things like sticking callipers, brake shoes, damaged rubber hoses and last of all do a complete brake fluid change to ensure you don't have something in the brake lines.
Hope this helps, it should take less than an hour to do all of the checks.

Thu Mar 22, 2018 11:56 am
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GTM on order

Joined: Tue Nov 28, 2017 12:04 am
Posts: 4
Post Re: Please help advice on a brake fault gtm coupe
Thanks chaps, run through the suggestions and check servo connections just in case I'd reversed them but still the same I even tried jacking the car to get a good angle on the servo ( somebody also suggested this helps the bleed).

I can only conclude the new master maybe at fault, I've a new one coming and two new rear slaves.

Yes your right this type of fault is is maddening.

Mon Mar 26, 2018 9:06 pm
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Joined: Fri Sep 14, 2007 10:34 pm
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Post Re: Please help advice on a brake fault gtm coupe
Another possible is the master cylinder does not have enough volume to move the fluid for you setup. It could even be that your pedal is bottoming out on something before you have reached full travel on the cylinder. worth a check if you are confident there are no leaks/air in the system.


IF wile e coyote had a GTM he would have one with Nitrous too! watch out road runner! The First GTM into the 11s then the 10s, PB 10.87 @ 125.5 Mph on A048 tyres, and fully road legal at Santa Pod

Wed Mar 28, 2018 10:01 am
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