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MG Midget and Sprite Technical - brake pedal travel
|Our 1968 Sprite with a stock braking system seems to have a lot of brake pedal travel. I recently rebuilt the front calipers and replaced the flex lines with new moss rubber flex lines and the green stuff pads have very little wear. It has been bled 3 times at all 4 corners. The rear drum brakes are adjusted as tight as I dare feeling resistance to turning the wheel by hand on each side. There is about 1/2 inch max free
play before the pedal starts to act but that is not adjustable. Then the pedal travel goes 90% to the floor but the braking is good. I know the problem is not adjustment of the rear shoes. Is there a problem place to get bled in the front. The brake lines drained while I was rebuilding the front calipers.
|BEW Brett Wright|
Are the discs in good condition - i.e is the run out within tolerance?
I bled my brakes to death before I replaced my discs & job done.
|Bleeding can be tough just keep trying.|
Mixing pedal bleeding and and an easy bleed should do the trick.
The free play sounds a bit big to me, please check all clevis pint and holes for wear.
If you keep experiencing a long pedal after these things check if you have not got a mis match in rear wheel cylinders and MC
|Gavin's comments about the front disc is very relevant. If the run out (wobble) is excessive as the disc turns it kicks the pistons back so there is then tooo much travel needed befor the brakes come on. Typified by a second press on the pedal when the car is stationary having a much shorter stroke.|
Some of the cars around 1968 have an adjustable push rod on the M/Cyl.
Too much free play at the pedal can also be from the pedal pivot point wearing oval, but yours doesn't sound too bad
|Agree about the pedal pivot point. It is fabbed up from metal which is far too weak and thin to withstand the considerable forces acting upon it, and any ovality here will be magnified many times at the foot end of the pedal causing far too much free play. I had to re-inforce and sleeve mine - worked a treat and has virtually eliminated any play.|
|I will check the disc run out. They are brand new but that doesn't mean they are true. The play in the pedal is from clevis wear. I could go after that and gain a bit. I need to check all 3 flexible brake lines for inflation too. That could cause some pedal travel. Maybe I should bleed more. I fear the master cylinder got really low on the front portion when I had the calipers off. Maybe I should do a MC bleed.|
|BEW Brett Wright|
|Disc run out can be caused by corrosion or flecks of rust on the mating surface between the hub and the new disc. Unless you were careful about leaning it up really well. It doesn't take much.|
|Thanks for your patience. Yesterday I was able to check the rotors for run out. It was negligible (within a few thousands). I measured the flexible lines at hard brake pedal and relaxed and there is barely 1 thousandth of an inch expansion. Then I bled the master cylinder by cracking the exit line half a dozen times. This resulted in some improvement. I then re-bled at the caliper without any change. Pedal arm stops at least an inch before the firewall now. I may still have some air that I might get out with a power bleeding approach and I can still repair the linkage slop at the clevis. I am just surprised that the bleeding is so ornery/tricky. Keep in mind I likely drained the front half of the master cylinder by inadvertent syphoning while removing the calipers for rebuild.|
|BEW Brett Wright|
Drive it! A fter 100 miles or so, preferably on back roads, methodically bleed it again, starting at the back. Road vibration during use of the car will cause microscopic bubbles in the fluid to coalesc together and then they can be properly bled out, giving a firmer pedal.
|If the calliper pistons are not moving freely enough through the seals, the seals will grip on them and compress, acting like a return spring. This will retract them too much and cause exactly what you are experiencing. It takes only 15-20 thou of retraction per piston and most of the stroke of the master cylinder gets used up getting the pads up to the disc. Solved by removing the pistons and applying more lube before assembling.|
|Agree with Paul|
also just to elimiate the rear you can clamp the rear flex hose from the body with a line locker just to make sure the problem's not back there
Or also to check the front calipers you could linelock the front flex hoses and if it is the calipers causing the problem the pedal should go hard with the locks on
This thread was discussed between 12/09/2017 and 21/09/2017
This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.