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MG Midget and Sprite Technical - Brakes, long pedal travel.
|Right, so it seems like I still can't get my brakes to work properly..|
Car is frogeye sprite with disc in the front and a sleeved dual master cylinder to 3/4", Everything is new brakelines, hoses, wheel cylinders, callipers, master cylinder rebuilt. No leaks and the pipes and air nipples are fitted correctly. Clutch works as it should.
The cars stops alright but the travel of the pedal is really long and stops just about 2-3 centimetres from the floor, if i pump the pedal again it travels just half that and is completely stiff and stays where it is until released. Air in the system you all probably assume? I've bleed the brakes with a pushing type bleeder with a pressure of 2kg, I've even bleed the brakes pushing fluid from the nipples into the master to make sure all the air is out. I've bleed the brakes the old fashioned way and bleed them with a airpressuerd suction bleeder. So my question it what did i do wrong?
I've loosened the callipers so I could bleed them in angels to avoid air pockets in them. Has this something to do with me changing to discs in the front and having regular drums in the back? The master cylinder is sleeved and fitted with 3/4 pistons. The rear cylinders appear to be fine no leaks on the inside of the drums but can they still be the culprit? I've read somewhere that it's preferable to use morris minor wheel cylinders when converting to disc so are my current ones of the wrong fluid area?
It's either a faulty master or rear cylinder, a incorrectly assembled calliper so something is blocking something, or it might still be a leak in the system where it draws air in but not letting fluid out..
Am thinking of disconnecting one wheel at a time and plug that line and bleed the system again, if I get correct movement on the pedal I can isolate where the problem is. One option is to switch the output on the master and if the brakes work correctly on the clutch port, then it's a faulty master..This is all very messy though.
I've spent a lot of time on the car and would really like it finished soon, but the motivations is at it's lowest having to redo and check every work twice or more.
Anything would be helpful.
|Alexander Sorby Wigstrom|
On the first pedal stroke, even though it is long, is the pedal firm, or springy?
If its springy, then maybe there is still air in the system, If its firm, but close to the floor then I would suspect there is too much clearance on the rear brake shoes and they need better adjustment. Be sure to do this with the handbrake fully released - or even disconnected.
The other thing worth checking is that the pivot point for the pedal isn't worn oval so that although the pedal moves a long way, the pushrod to the M/C isn't moving as far as you think.
Thanks for the response, the pedal is firm on the first stroke. I disconnected the handbrake and adjusted the brake pads as far as they go so that the brakes was fully engaged before i bleed them and then adjusted them back could it be as simple as one of the brake adjusters not engaging properly and releasing the pads to far when you release the brake?
I'm not sure what you mean with the pivot point (language) but the pushrods are new and at they are of the correct length for the cylinder. But I need to check how far they travel when the pedal is pressed.
|Alexander Sorby Wigstrom|
|I did this on our Frog but replaced the backplates and cylinders with 1275 midget type.|
It is recommended that MM rear cylinders are fitted to retain the Front/Rear brake balance.
If the rear brakes are the culprit you can usually put your foot on the brake peddle and operate the handbrake. If you can feel it through your foot it's rear brake travel. Operating the footbrake with the handbrake fully on and it feeling different is another sign.
Have you swapped the front calipers? (It's obvious I know but I've done it)
|Rob aka MG Moneypit|
|The callipers are fitted correctly.|
Will check, the clearance on the back once again. Or might as well order different wheel cylinders and check the fitment of the springs and adjusters in the rear then as well.
|Alexander Sorby Wigstrom|
So if the pedal is firm on the first press, but just goes down too far then it is unlikely to be air in the system.
Sounds like you have the brakes adjusted correctly.
It could perhaps be a worn pedal pivot / fulcrum / lagertapp ?
|Alright now i get what you mean, but if that were worn would it affect the travel that much considerd that the travel is less than half as long on the second stroke?|
|Alexander Sorby Wigstrom|
|I don't know the maths of it, but a fairly small displacement at the fulcrum, will loose quite a lot of movement in the leverage - the ratio between the length of the pedal side and the pushrod side of the fulcrum.|
|You haven't mentioned how much free movement you've got at the pushrod when the m/cylinder is relaxed. I adjust it so there's only a couple of mm of lateral play before the hydraulics start to operate - you have to feel this with your fingers. Then the pedal should start to work after about an inch of travel. To an extent brakes do self bleed over time but you can help the process by leaving the brakes wedged on and pressurised over a weekend. If you also dust all joints and nipples with talcum powder (one for Prop no doubt) you'll also easily see if you have the slightest weep of fluid. Make sure the rear shoes are properly centered - thumping the drum with a rubber mallet as the brakes are applied can help, but again after a period of bedding in they will need adjusting again to dial out any slack and subsequent long pedal.|
|Alex you need to clamp off all the flexible brake pipes,(using brake pipe clamps)then try your pedal, if you now have a good pedal, you will know your problem is not the mater cylinder, or air between that and the flexible brake pipes, next get some one to sit in the car and feel the pedal, release the clamps one at a time checking the pedal movement after each removal, if the pedal drops after a removal then you will know which wheel your problem is in, and if you still have a problem with all the flexibles clamped off then your problem is between the flexibles and the master cylinder,this is the best way I know to prove where your fault is, as it eliminates each component in turn and cuts out the guess work and renewing parts that are not faulty, this method of testing cant be used if you have Goodridge type braided hoses, as you cannot clamp them to cut off fluid movement, A.T|
You need to determine which leg(s) of the brakes is giving the problem. Use a process of elimination. Method:
(i) With hose clamps (or modified mole grips), clamp off the two front hoses and the rear hose.
(ii) Operate the brake pedal. Do you still get the fault? If so, the problem lies with the BMC or pipework before the clamps.
(iii) Remove one hose clamp in sequence, each time operating the brake pedal afterwards. The process should identify which leg(s) is giving the problem.
(iv) If the problem lies somewhere with the rear brakes, you can remove each drum in turn and clamp the wheel cylinder so that it doesn't operate. Again, try the pedal.
Aside: you might have been supplied with wrong new parts. If you have a vernier measuring calliper, check the inside bore of the rear cylinders.
|To centre the shoes I do as Fergus says and then also get the attractive assistant to gently footbrake as I am moving the rear wheel by hand while jacked up. I adjust up between thumps and rotations until no more adjustment is possible. Then I know they are as near as I can get to being properly adjusted without a road run like when I run the MoT out.|
Before assembly I ensure the back plate and edges of the shoes are clean, flat and have a tiny smear of copper grease on the back plate rub area and the edge of the shoe to ensure free movement.
Hope this helps.
|If you do change the rear cylinders use later morris minor ones (up to 1971) with a 3/4" bore. The frogeye and early morris minor ones up to 1962 were 7/8" bore|
|Remember too that it can take 200-300 pumps of the pedal to get new seals to bed in properly in M/C bores and this can give grief when trying to bleed. It is a relatively recent issue which must be due to synthetic rubber used in the seals.|
It was pointed out in a previous issue of the MG club magazine, and it did happen to me a few years ago when fitting a new M/C. Went through my usual bleeding procedure which always works without fail, and was getting nowhere and I could feel the seals were just not bedding in, so I sat in the car for a few minutes and pumped the pedal many, many times, hundreds of strokes in fact.
Then tried the bleeding and it worked as expected.
May not be the problem in this case but it is something to remember, if bleeding with new seals is causing problems.
Mine has worked perfectly ever since
|A lot of good replies, My hoses are steel braided with a teflon tuben inside so i can't clamp those unfortunately. Clamping the rear cylinders though I can do. Small steps.|
Thanks for all your input.
|Alexander Sorby Wigstrom|
|A bit of a long shot...|
Could it be something like the calliper seals causing the pistons to retract into the callipers? Can you see if there is any gap (or if the pads are loose) before the first press on the pedal (when you might expect to get your long pedal travel), and whether the gap has gone after you have pumped the pedal? If this is what is happening, maybe it might sort itself out once the brakes have bedded in and the pistons are 'resting' at a slightly different position.
|On that last point, it may also be worth checking the disc run out with a dial gauge. If they are warped, or have excessive run out then that will push the caliper pistons away from their proper close contact position against the disk. It then takes more fluid pumped, = longer pedal stroke - to bring the pads back into contact with the disc.|
|I think this a good example for not using braided Teflon hoses, which I personally think are a complete waist of money on a road going car,|
|Can't clamp them, but the brakes feel loads better.|
|Check simple things like wear in the clevis pin at the master cylinder, a small amount of wear in the moving parts is magnified to cm's at the pedal|
Really good advice in this thread so far.
The Morris Minor 3/4 inch single rear brake cylinder that fits Frogeye Sprite rear brake back plates is for the 1098cc Morris Minor, part numbers: GWC1116 and Lockheed number LW11287.
Dave on another thread - Pedal Box Adjustment - offered this advice on adjusting the brake pushrods on the master cylinder, which goes rather well with Fergus's advice above:
1. Slacken the adjuster nut on the m/c push-rod.
2. Set the length of the push-rod to give a free movement of the pedal pad of approx. 5/32" (4mm) before the m/c piston begins to move. the push-rod must have a minimum of 1/32" (.8mm) free movement before the piston begins to move.
PS Do not give up - I have been having similar problems!
|Alex, just a thought have you checked the internal dimensions of the rear brake drums? If they are older and either worn or been skimmed too far then there will be a long pedal travel before the shoes fully engage with the drums sometimes even when fully adjusted. Drums can also wear so as to be tapered making accurate adjustment difficult.|
|Jonathon's long shot definitely does happen, I have found it more than once. It only takes 20 though of spring-back and you need almost the full stroke of the pedal to get the pads back to the mark.|
Solution is to dismantle the caliper, clean everything and lubricate the seals & pistons.
|(sigh) I see fat fingers goofed again, make that 20 thou in case anyone can't work it out.|
This thread was discussed between 01/11/2016 and 17/11/2016
This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.