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Author: Subject: How do you measure the length of pushrod for brake master cylinder
novicebuilder

posted on 13/11/13 at 12:09 PM Reply With Quote
How do you measure the length of pushrod for brake master cylinder

I expect it is a stupid question but I need to know how to measure the length of the brake push rod for the master cylinder.

I have made the bracket and welded on the push rod with a long length on it, now I need to cut it to size.

I have mounted the master cylinder on the back of the pedal box, with no brake fluid in it and the piston appears fixed as it won't move in when pushed with the rod. Is this the maximum length of travel or is there something else I need to know to get it correct length. Do I have to fit it when there is fluid in the circuit? How exactly do I measure the length to accommodate all pedal movement?

Grateful for guidance from the experts, please.
Thanks.

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britishtrident

posted on 13/11/13 at 12:25 PM Reply With Quote
The rubber grease at the end of the cylinder bore will have congealed Get some fluid into the cylinder and give the piston a good shove to free it off.

The critical part is piston in the cylinder must come fully back against the end stop in the normal brakes off position, as even a fraction of a mm movement can cause the brakes to bind as they warm up.

[Edited on 13/11/13 by britishtrident]





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jps

posted on 13/11/13 at 03:02 PM Reply With Quote
Not that this helps you - BUT: I expect this is why the pushrod I took out of my Sierra donor had a threaded insert in the end - so it could be minutely adjusted in length...

The Haynes Roadster pedal box design has an adjustable stop under the brake pedal - which I assume is also designd to allow the pedal to be 'adjusted up' to the master cylinder - rather than the tolerance being made up in the pushrod itself...?

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britishtrident

posted on 13/11/13 at 04:17 PM Reply With Quote
The tandem master cylinder has internal stops on both piston.

However if you want the car to stop without needing the assistance of a Sumo Wrestler to press the pedal the first thing to do is ditch the Sierra master cylinder and fit a smaller bore one.
The Sierra master cylinder is sized to work with a much higher pedal leverage ratio and a servo. Even though your car will be 1/3 the weight of fully laden Sierra with the Sierra master cylinder you would need push the brake pedal very hard indeed to stop the car.

[Edited on 13/11/13 by britishtrident]





[I] What use our work, Bennet, if we cannot care for those we love? .
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Macbeast

posted on 13/11/13 at 05:15 PM Reply With Quote
I'm nervous about contradicting BT, who always looks as if he knows what he's talking about, but I used the unmodified Sierra M/C without the servo and the braking effort is fine. The only problem was that there didn't seem to be any internal lip for a circlip to retain the pushrod so I had to fabricate a crude endstop on the brake pedal itself to stop it flopping about.





I'm addicted to brake fluid, but I can stop anytime.

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novicebuilder

posted on 13/11/13 at 07:52 PM Reply With Quote
Thanks.

When you say put some fluid in. Do you mean fill the reservoirs with brake fluid and then push the piston with the push rod so any congealed grease is loosened and the piston becomes free and bounces back to fully extended position?

If this is correct then I presume when the piston has bounced back, that is the brakes off position? So I would then measure the piston rod length from brakes off pedal position and cut to size. Will it then be correct length to push into piston fully and apply brakes fully?

Finally is there any other kind of seal needed around the push rod as it is just sitting in the chamber with a mm clearance.

Sorry to be so naive but this is my first build so each of these tasks is completely new to me.

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Peteff

posted on 13/11/13 at 08:21 PM Reply With Quote
quote:
Originally posted by Macbeast
I'm nervous about contradicting BT, who always looks as if he knows what he's talking about, but I used the unmodified Sierra M/C without the servo and the braking effort is fine. The only problem was that there didn't seem to be any internal lip for a circlip to retain the pushrod so I had to fabricate a crude end stop on the brake pedal itself to stop it flopping about.


I too used a servoless Sierra M/C with Cortina front and Capri rear brakes and it worked fine with nice feel to the pedal. You need to get the pedal pivot ratio sorted if it's too heavy. I had a light spring holding the pedal and a stop as well.





yours, Pete

I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.

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AdrianH

posted on 13/11/13 at 10:07 PM Reply With Quote
If you are using the standard Sierra Brake set up say vented disk at the front and 9 inch drum at the rear the the standard Sierra MC is fine The Haynes Roadster pedal set up gives about 6.5:1 leverage and I measured 20kg on the pedal to get full lock-up. Heavy braking is before that and I get a good feel back from the pedal, in that it is easy to back off.

If you are using 9 inch drums you may wish to consider dropping the rear slave cylinders down from from the standard 22.22mm to 19.05mm as used in some of the older escorts, same fitting and external dimensions. As said dependent on thing the Standard Sierra vented disks and 9 inch drums. This brake set-up gave me a lot better brake balance, left the handbrake alone to give good results with that on the IVA and meant I did not need to use any restricter in the rear line.

On to the length of the push rod. As mentioned get some fluid into the cylinder and get it moving, the maximum travel is 30mm I believe so the push rod and pedal travel needs to be enough to allow the pushrod to go in by that amount.

So with a rod resting just in the cup, of the MC measure to the end of the cup to give a base line, give another 30mm at least say 32/33mm then measure the distance away from that line to the pivot point. You may well need to drop the stop bolt underneath the pedal stop so the pedal drops more towards the drivers seat.

I also hope it was a typo when you said you have welded the pushrod on, it should not be welded to anything. The back of the Haynes pedal has a hole in it, get a longish 6mm 8.8 bolt, chop the hex head off it round it off nicely, and use that with one of these eBay Item you can get different lengths to suit various pedals. Use the threaded section of the 8.8 bolt a 6 mm locking nut and it will allow some adjustment in and out of the clevis pin. And most importantly alow it to pivot and keep the push rod in the end of the MC cup. When set up with the pedal at rest there should be a little free play between the push rod and the end of the cup but not enough for it to come out of the cup.

The main thing is the pedal should not be hitting the bulkhead before the MC reaches full travel.

Adrian





Why do I have to make the tools to finish the job? More time then money.
Build diary at http://www.tamarisktechnicals.com/pages/roadster.html

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davidimurray

posted on 13/11/13 at 10:55 PM Reply With Quote
I agree with everything Adrian said!

Followed his advice and changed the rear drum cylinders for the 19mm and I sailed through iva.





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novicebuilder

posted on 24/11/13 at 05:32 PM Reply With Quote
further guidance on Haynes roadster donor master cylinder set up

I have managed to free the master cylinder piston and it does travel in 30mm as described. So I can now measure the length to cut the push rod.

I understand from what you have said that the pushrod must be fully back in 'brakes off' position. However, there is only 35mm of travel in the brake pedal between floor and bulkhead. So to have the push rod fully extended the brake pedal nearly touches the car floor and the position of the pedal looks very odd? Also there is not room for the floor stop to have a nut each side of the floor as described in the book, because the floor stop will then be too high?

Have I got something wrong here, or is it meant to be like this?

Also does the pedal really have to travel the full 30mm for full braking; it seems a long way to move the pedal.

Hope this makes sense.

Grateful for more guidance.

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britishtrident

posted on 24/11/13 at 08:01 PM Reply With Quote
You don't need any stop on the floor, a tandem dual circuit master cylinder has internal stops on both the primary and secondary pistons. If it didn't it wouldn't fail-safe in the event of one circuit failing.

On a non-servo brake system if its correctly bled and adjusted there should only be a tiny amount of movement of the mastercylinder pistons. When a most drivers drive a non-servo system for the first time they get a bit of a culture shock because the pedal (should) hardly moves and the it needs a lot of pedal pressure to stop the car when compared to the grossly over-servoed brakes on modern cars.

You really get a new master cylinder (do a Forum search on Fiesta or Fiat mastercylinders) rather than take a chance on a Sierra cylinder that is yonks old for which overhaul parts are probably unobtainable.

[Edited on 24/11/13 by britishtrident]





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davidimurray

posted on 24/11/13 at 08:39 PM Reply With Quote
The stop on the floor is needed so the pedal doesn't fall back and the pull rod come out of the m/c.you also need it to adjust the frreplay between the rod and mc when released. I welded a nut to the front of the Haynes brake pedal and then put a bolt through it so you need a single locking nut.

Set the pedal up so you can use the full mc stroke. I did not initially and found the pedal used about 20mm of mc travel, but if you pressed hard you could just touch the bulhead. This does mean the pedal is a long way back but it works nicely. If you have a deep clevis you can re-drill the pedal hole further back to bring the pedal more vertical but again you may be limited by the pedal hitting the bulkhead.

If you want any pics or dims of my haynes setup with standard sierra mc just drop me a msg.





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britishtrident

posted on 24/11/13 at 10:55 PM Reply With Quote
quote:
Originally posted by davidimurray
The stop on the floor is needed so the pedal doesn't fall back and the pull rod come out of the m/c.you also need it to adjust the frreplay between the rod and mc when released. I welded a nut to the front of the Haynes brake pedal and then put a bolt through it so you need a single locking nut.

Set the pedal up so you can use the full mc stroke. I did not initially and found the pedal used about 20mm of mc travel, but if you pressed hard you could just touch the bulhead. This does mean the pedal is a long way back but it works nicely. If you have a deep clevis you can re-drill the pedal hole further back to bring the pedal more vertical but again you may be limited by the pedal hitting the bulkhead.

If you want any pics or dims of my haynes setup with standard sierra mc just drop me a msg.



You are not actually driving around without the master cylinder push rod being held captive to the master cylinder?

The push rod should have mushroom end and be held by washer and circlip.





[I] What use our work, Bennet, if we cannot care for those we love? .
― From BBC TV/Amazon's Ripper Street.
[/I]

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davidimurray

posted on 25/11/13 at 02:05 PM Reply With Quote
The master cylinder piston has a recess in the end that the piston rod sits inside, this is a parrel bored hole.. The mc piston cannot come out because it is held by a circlip and washer. When fully released, the mc piston rests against the washer. The recess in the mc piston is a smaller diameter than the washer. Hence the push rod is attached to the pedal and the end of the rod rests inside the recess (which from meory is about 10-15mm deep). This is basically the same setup as with the Sierra Servo, as the end of the servo simply rest inside the recess.

There is no way you could restrain the rod inside the recess as it is a parrallel bore. The only possible way would be to have a top hat that sits behind the washer, but that would then compress the piston slightly and block off the reservoir ports.





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Dick Axtell

posted on 25/11/13 at 03:27 PM Reply With Quote
quote:
Originally posted by davidimurray
This is basically the same setup as with the Sierra Servo, as the end of the servo simply rest inside the recess.


Definitely NOT!! The Sierra mcyl had no internal piston backstop, because its released position was determined by the servo output rod. This was always engaged with rear of the mcyl primary piston, thereby determining its position.

If, however, your mcyl has e.g. an internal circlip retaining the pistons within the bore, then it was intended for non-servo use.





Work-in-Progress: Changed to Zetec + T9. Trying to sort out carbs.

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davidimurray

posted on 25/11/13 at 03:56 PM Reply With Quote
My MC was from a 1987 sierra with a servo and uses the girling m/c - It was definitely servo'd as I stripped the car and replaced the m/c with a new identical unit.

The drawing below shows an exploded view of the Master cylinder - note the circlip on the right hand end. You can also see Pin F which is the internal stop pin. If you remove the circlip the piston will not come out unless you remove pin F



Without pin F and the circlip the piston will pull out but with either of these in situ it won't. The front and rear pistons are acctually seperate unit but connected via a long internal bolt.

I suspect there may well be different m/c design but this was the setup on mine.





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