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 Posted: Dec 10, 2022 06:46AM
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There is play within the connection between brake pedal and the master pushrod.The master is less than a year old so I don'tbelieve there is blockage. Not certain about check valve. I did notice the wheel cyl pistons do not have hole to attach retention springs. I guess that the hole is only offered with the OEM parts. I plan to, hopefully, drill holes in situ and go from there. Again then, I value your support. Will keep you posted. cheers

 Posted: Dec 9, 2022 10:28AM
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I dont think there should be a gap between the piston and the shoe, it implies the piston is retracting into the wheel cylinder on its own, which is odd. If im not mistaken there is supposed to be a check valve integrated into the master cylinder that holds 1-2 psi of line pressure to prevent this from happening. I know there are aftermarket in-line devices that do this as well, and it may help to add one. If the fluid intake port on your master cylinder is dirty or blocked, it can cause fluid to be pulled back out of the wheel cylinders instead of from the fluid reservoir. Also, make sure the master cylinder pushrod can completely return to its "off" position. It is possible that the pushrod is bottoming out on the brake pedal before it reaches the bottom of its stroke, which can also cause what you're describing. This usually only happens if the brake master is mounted directly to the bulkhead without a plate or gasket. The thickness of the mounting gasket is usually enough to prevent this from happening. 

 Posted: Dec 9, 2022 06:55AM
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Hello again, This stuff interrupts my sleep. Ref. gap and pedal travel. I noticed that the receiving end of the pistons there is a hole in one side of the slot where the shoe rests. My old brake setup did not have the springs installed. I revisited my parts sources and discovered this excerpt from MiniSpares: "If you have to pump your brakes, check that these retainer springs P/N 27H3753 are attached to the wheel cylinders and brake shoes properly". I came across these springs earlier while researching parts and was advised they are not necessary. Their primary function is to pull the shoes against the backing plate. But if the two return springs are installed correctly, the retainer springs are not necessary. This bit of info corroborates your advice ref gap between piston and shoe. I plan to order the retainer springs soonest. cheers

 Posted: Dec 8, 2022 03:14PM
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Thanks for the reply. All springs are behind the shoe. There is a little gap between the shoe and piston. Can't manipulate boot. However I can back the shoes off a bit so they will eliminate shoe to piston gap. Unfortunately, all I have done is to relocate the gap to drum /shoe. I have ordered a 2 stage master. When all is done, I will update. Cheers

 Posted: Dec 8, 2022 05:23AM
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CA
One other thing to check re the low pedal problem is the position and freedom of the dust boots on the wheel cylinders. If they are sticking or stuck to the piston shafts, they can act like springs and slowly pull the pistons back from the shoes.  I had this happen and if I recall it might have been on a new cylinder.

Try and remove the drums without easing off the adjustment. See if any of the pistons have pulled back from the shoes at all. Look at each boot and push it in along the piston shaft so the springiness of the boot pushes out not in.

.

"Hang on a minute lads....I've got a great idea."

 Posted: Dec 7, 2022 03:06PM
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ah, thanks for the clarification, after reading over your original post I thought that might be the case. I don't think I knew they were still using 4 wheel drums in 1980. 1 silly silly issue I will assume you have not made, is the brake shoe return springs. I have seen on 1 occasion, someone install all of the return springs on the front or outboard side of the shoes, which pulled them outward away from the backing plate. They always sounded and felt like they were adjusted correctly, but caused an almost completely dead first jab at the pedal if the car was in motion, and then an ok stop after pumping the pedal again quickly. I think that may have been the only 4 drum car I've serviced and we ended up upgrading to front discs anyway. As far as I can tell though your system should work. Up till 76 most cars used a 17.8 single circuit system anyway. If all of the shoes are adjusted correctly, all of the springs are in correctly, and there is no air in the system, I don't see why your current setup would not work. 

 Posted: Dec 7, 2022 12:28PM
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thanks for correction. GMC 167 is for diagonal. GMC 227 is stepped bore for front to rear. Based on the issue at hand, is the 227 my only option? I fear it is. I value your advice and feedback. Cheers

 Posted: Dec 7, 2022 12:13PM
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Thanks muchly for the reply. 1) drums all around. 2) twin leading shoes. 3)rear cyl 3/4 (19.05 mm) 4)front cyl 15/16 (23.81 mm) My theory: The stepped master bore pushes a greater volume of fluid than the straight master. However I could be wrong on some or all counts. Looking forward to reply. Cheers

 Posted: Dec 5, 2022 01:22PM
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I think you may have your masters mixed up. The 227 stepped bore is for a front/rear split, not a diagonal. Sending different pressures to diagonal corners would make for some very wonky braking. The larger rear bore makes for quicker pedal engagement and sends less pressure to the rear where it isnt needed as much, and the smaller bore sends more pressure to the front for the discs. Your gmc167 17.8 bore (intended for diagonal split) will need slightly more travel to send the same volume of fluid to the rear drums. that said it should still work just fine if everything else is in order.  What front calipers are you using and what is the bore of the rear wheel cylinders? If you're absolutely certain there is no air in the system, check for play in your front wheel bearings, as it will cause the same symptoms as you're describing. One pump of the pedal is needed to straighten up the rotor, and the next actually provides clamping force. 

 Posted: Dec 5, 2022 11:57AM
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For my 1980 right hand drive mini with split front to rear: Will the GMC167 with 17.8mm bore function properly for said Mini? If not, then why. Be advised the GMC 227 stepped bore 17.8mm/19.0mm is engineered for the diagonal split system. A new GMC 167 is currently in said mini. But I have to pump the pedal twice for the brakes to grab effectively. One pump, and the pedal goes to the floor with minimum effect. All front wheel cylinders and shoes are new. System bled. The original master had 11mm fittings and had the black band with switch on side of the body. All shoes are adjusted to 'just scraping' the drums. I think I know the solution, but I'll for response. Cheers,