MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Brake pedal long travel (NMC)

I apologise for posting this here but there are quite a few correspondents on this board who have vast experience with all types of cars and I would like to tap into that gold mine if I may. For me working on cars has always been a hobby, secondary to work, home, raising a family etc.

My MGB V8 has a full Hoyle suspension set up, front and rear. http://www.hoyle-suspension.co.uk This comes with 4-pot front calipers made by Hawk, and Ford Sierra/Granada single pot rear calipers (which are only just visible in the photo). The master cylinder is a standard late model MGB dual circuit type with the servo in line with the master cylinder. This is what Hoyle recommend.

The brakes work alright but have a long pedal, which when you have been driving a modern car can be alarming initially. They are also considerably under-servoed compared to a modern. I have bled them several times without improving them but I can't eliminate the long travel pedal. They don't pump up so it doesn't seem to be air in the system or the pads retracting too much.

Has anyone got any ideas what could be the cause? Might it be to do with the peculiar valve thing in the master cylinder, the function of which is a mystery to me.






Mike Howlett

I had long pedal travel with my new calipers on the midget. Fixed it by holding the pedal down, against pressure for 24 hours (use a piece of 4x2 on the pedal against the cross member in front of the seat. I believe this was caused by the caliper seals not positioning correctly, therefore the calipers retracted too far when the pressure comes off.

There is a good round of this about a Porsche somewhere on the interweb.

All that said, you say the brakes don't pump up...to a shorter stiff pedal...
Tim Carter

Have you checked for run out on the discs with a dial guage?
GuyW

No I haven't, but the long pedal is still there even if the wheels haven't turned at all. I think I may try removing the brake pads and replacing them with a wood spacer that's slimmer than the pads. Then I can blow the pistons out of the calipers a bit before pushing them back, just in case there is any problem with a piston not working properly. The rear calipers are brand new, so I'll concentrate on the 4-pot front calipers which haven't been touched for 15 years.
Mike Howlett

Hmm-IF you've been bleeding both inner and outer front bleeders, then I'm thinking rear calipers
Try this-
1) detach the handbrake cable
2) depress the foot brake pedal several times (at least 10 or more good applications) or until all piston adjustment is complete (solid pedal).
3) reattach the hand brake cables
4) adjust the handbrake cable tension until the lever on the caliper is between 0.5-1.5mm(MAX) off the stop.

Got a good pic of your master cylinder with the 'valve'

Have you got the valve switch wired up--
The valve is a piston that moves under pressure, if one end of your brakes hydraulics isn't operating properly the piston is pushed accross by the good side cutting fluid flow to the failed side

Have you got the plumbing correct to the cylinder

When bleeding these I find it best to bleed the rear first then the front

willy
William Revit

It's possible the hydraulic ratio between the slaves and master are giving you the long pedal action, have you asked Hoyle? You might need a large diameter size of master cylinder.

Also is the long travel all actuation or is some travel taking up the slack? It is possible to make an adjustable brake rod (by threading the end of the rod and fitting a threaded clevis fork to replace the standard one).
Daniel

All good suggestions above - quite a number of possibilities.
A couple of things to try and narrow it down:

If there is excessive pad retraction, it results in a measurable clearance increase between pad and disc. Check with a feeler gauge. 20 thou is all that's needed to almost lose the pedal completely. Normal brakes we have measured seem around the 4-5 thou mark. I'd be interested in what others have found normal clearance to be.

Try and isolate where the problem is by carefully clamping the brake hoses. (Carefully = avoid sharp edges clamps and excessive pressure.) Clamp all hoses, if the pedal remains long, the problem is in the mcyl or air in the pipes. If the pedal goes more or less solid (as it should), then release the clamps one by one, checking the pedal each time to find which corner/end makes the pedal go long.
Paul Walbran

Thanks gents. To answer Willy, here is a couple of shots of the master when first installed 15 years ago. I never bothered to wire up the switch as it seemed to me that I would know immediately if one circuit had failed.

I'm not sure I understand what you mean when you write "adjust the handbrake cable tension until the lever on the caliper is between 0.5-1.5mm(MAX) off the stop." I am posting a photo of the handbrake lever mechanism in the next thread. The cable pulls down on the hook against the spring. I've no idea how it operates the pads.

Lots of things to try, but my garage is taken up with my VW Touran 1.6 DTi at the moment which has decided to leak diesel fuel from somewhere around one of the injectors. I suspect the fuel return pipes and have ordered a Gates set.






Mike Howlett

The Ford Sierra rear caliper.


Mike Howlett

OK
I don't know what setup you have for a handbrake cable, but the general idea is to slacken the cable adjuster off enough to get the eyes on the end of the cable to be completely free/loose on the hooks on the caliper so that there's no preload on the lever., then do the pedal pumping to set the piston adjustment then readjust the handbrake cable so that it's 'just' putting a tiny preload on the lever(s) without hardly moving the lever.
There is also a possibility that you might have tripped the piston in the master cylinder while bleeding the brakes. With the switch not wired in you'll have no easy way of knowing--you'll have to remove the switch and get a little screwdriver up in the hole and slide the piston accross until the deepest part of the groove in the piston is lined up with the centre of the hole, you'll be able to feel the groove with the screwdriver or poke a mirror in there for a look-- then refit the switch. No fluid will come out the switch hole except maybe a tiny residual drip or two--

Is this a new problem or has it always been like it-
If it's new have you changed something on the car that has changed things--

William Revit

No, it's a problem I have always had since I finished the car in 2009. But buying a small Honda for my wife with massively over-servoed brakes has highlighted to me the long pedal on the MGB.

Thanks for the heads-up about the master cylinder switch. I have never touched it, but I will have a look now. And thanks for making the adjustment of the handbrake clear. In fact that's what I have done with the new calipers. The guys that test the car each year have always commented that the handbrake only just passes the test criteria. It can hold the car on a slope, but as an emergency brake it is pretty near useless. I have always thought the leverage applied by the MGB handbrake lever can't be as great as that on the original Ford model.
Mike Howlett

Mike-something to try-
Pump the brake pedal a few times(engine off)to dump all the vac. What does the pedal feel like now.
It should have the normal 1/8" of free travel then pushing further should get solid almost straight away without any secondry travel.
If it moves the 1/8" then has an extra travel amount then feels like it touches something then goes firm it 'might' need the booster piston rod checking/adjusting.

willy
William Revit

Mike

If this is a characteristic of the braking since you built the car (but now this is a less desired) then perhaps this suggests the need to experiment with different hydraulic bore sizes.

Any MGB V8 racers who can offer advice? What about Clive Wheatley the MGB V8 parts specialist? Does MG racer/car builder Malcolm Gammons still work at mg specialists Brown & Gammons, if so might be worth asking him.

Also the MG forum for V8 owners and builders might be worth a try. Does the book on such conversions have any handy advice?

Thanks
Mike
M Wood

Mike, it was the Roger Williams book I was thinking of: https://mgshop.co.uk/shop/books/5384-how-to-give-your-mgb-v8-power-by-roger-williams/ which might have some useful info and ideas on brake mods. If you don’t have a copy I can drop one round (might have an older version too).

There might be useful info here:
https://www.v8register.net/index1techsupport.htm

Cheers
Mike
M Wood

Thanks Mike Wood, but I have both the older and newer editions of Roger Williams' book. I intend to look first at the shuttle valve in the dual master cylinder, but a broken VW and other things are getting in the way!
Mike Howlett

Mike H. Your original question was whether you had excessive brake pedal travel on your car. I have seen a number of responses to the question, none of which recommend the, to me, obvious first step--have several people who have the rubber bumper cars measure the brake pedal travel on their cars. It may well be what you consider a long brake pedal travel is, in fact, the common length of travel necessary for proper fuctioning with that master cylinder.

As has been noted by others, the diameter of the chamber of the master cylinder has a direct effect on how much fluid is moved with any movement of the brake pedal. A larger diameter bore will more more fluid than a small diameter bore with the same movement of the brake pedal. While it has been something over 50 years since undergraduate physics, I seem to remember that a smaller diameter bore requires less force than a larger diameter bore to move the same amount of fluid. If my aged memory is correct, a master cylinder with a larger bore would take more force (pedal pressure) to stop than one with a smaller bore. But, the travel of the brake pedal would be less.

You seem to have some sort of focus on the secondary cylinder where the shuttle mechanism of the brake failure warning system is attached. If the rubber O rings in that part of the system are in good condition, it can have nothing to do with how the brakes work. If, however, the rubber O rings have deteriorated and they are allowing air into the primary braking system, they will have a definite effect on how the brakes work--you will have air in the lines. I would also expect that you would see brake fluid dripping from the switch (if you have used one), or the open hole as that is what happens when the rubber O rings deteriorate and allow air into the system and fluid to leak out. A very poor design, which often fails over time. It is worth checking out, but unless air is leaking in through the mechanism, it has nothing to do with how well the brakes function.

Les
Les Bengtson

To conclude this thread, I had only been working on the rear brakes, replacing the calipers and bleeding them. I decided to look at the infamous shuttle valve and unscrewed the switch. With a mirror I could see the shuttle was not central, so I gently moved back to the middle with a small screwdriver, and the brakes are much better.

The pedal feels reassuringly firm, and although the effort needed is more than on the modern Honda, they work well and are nicely progressive. I can only assume that working on the rear brakes and not touching the front brakes had caused the shuttle to move. I checked the switch with a meter and it is basically not working, so I left it out and replaced it with a short bolt just to keep any dirt out. The switch was never connected up anyway as there is no warning light on my dash.

Thanks to all who have contributed. The combined knowledge on this platform is amazing.
Mike Howlett

Mike---, the switch is a part of the dual circuit operating system, it's spring loaded pin is used to hold the piston from sliding accross randomly.If you're going to leave the switch out, I'd suggest you find a bolt long enough to allow you to grind it to a point on the end, so that when you screw it in it locks the piston in the centre position. --otherwise, if you happen to be bleeding brakes or changing pads or something ,there'll always be the posibility of the piston moving accross without you knowing and blocking off half of the hydraulics.
Normally the switch is a little bit spring loaded which helps hold the piston unless there's a pressure difference between the two halves of the system large enough to compress the switch and move the shuttle(piston) accross.
If you want to know how this piston normally works in the system i can run you through that if you like.
William Revit

Willie. While I hate to disagree with you as you do this for a living while I have only been a hobbyist for something over a half centurary, I have to disagree with you on the operation of the shuttle valve system. But, first, let us ensure that we are speaking of the same beast here. Mike notes that "the master cylinder is a standard late model MGB dual circuit type with the servo in line with the master cylinder." Thus, the standard master cylinder and servo unit used with the rubber bumper MGBs common here in the US.

Something over twenty years ago, having four rubber bumper MGBs at the time, I took an old master cylinder and sectioned it on my milling machine so that I could better understand how it worked. We had purchased an MGB for my daughter that had, four times, been into a professional MG Shoppe, in Tempe, Arizona, for problems with the brakes. Four times, the Shoppe had "rebuilt" the master cylinder without correcting the problems--the poorly designed brake failure warning system was leaking. I installed a new master cylinder and sectioned the original one to try to see why this model had the problems it did.

The shuttle is spring loaded to central position and cannot slide back and forth (as it is under spring pressure) unless acted upon by an asymetrical force (i.e. when one part of the system fails allowing the operating side of the system to force the movement of the shuttle). The switch is a "normally on" type with a ridge on the shuttle pushing it downwards, cutting off current flow. When the shuttle moves, it allows the switch plunger to rise upwards into one of the grooves on either side of plunger "collar" and that allows the electricity to flow causing the brake failure warning light to illuminate. Thus, grinding a teat on the front end of the bolt used to plug the hole serves not purpose.

It should be noted, due to the poor design of the system, that a deterioration of the rubber O rings on the shuttle mechanism, will allow air to enter the system and brake fluid to leak out of the switch and onto the inner fender. Thus, when replacing the switch with a bolt, a small amount of gasket sealer (or, perhaps, blue Loc-Tite) on the bolt will ensure that should the shuttle O rings fail, the hydraulic system cannot become contaminated with air.

Mike. Repositioning the shuttle should have no effect on how the brakes operate. The poor design of the brake failure warning mechanism means that it has no effect on the flow, or lack of flow, through the system. Have a sectioned example sitting right beside me as I write this.

Les
Les Bengtson

Well, I don't know the rights and wrongs of this, because although I have owned Midgets and Bs for about 40+ years, I am no engineer.

I have reread what Tim Carter wrote way back up the thread "I had long pedal travel with my new calipers on the Midget. Fixed it by holding the pedal down, against pressure for 24 hours (use a piece of 4x2 on the pedal against the cross member in front of the seat. I believe this was caused by the caliper seals not positioning correctly, therefore the calipers retracted too far when the pressure comes off. There is a good round of this about a Porsche somewhere on the interweb."

Having replaced the rear calipers (didn't touch the fronts) and then continued to use the car, maybe the use of the pedal has set things into their rightful places. All I know is that it is much better.
Mike Howlett

Les -- More than happy to be corrected- and apologize to all for the false info--
I'd completely forgotten that the design of this cylinder leaves a lot to be desired as far as not being a fully functioning dual circuit cylinder --It is dual circuit but doesn't have the ability to isolate a failing half like a proper dual circuit cylinder,--the shuttle valve in this cylinder is 100% as you say, solely for the purpose of turning on the warning light.
Again my apologies for the false info

willy
William Revit

This thread was discussed between 09/03/2025 and 22/03/2025

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.