|The MGA With An Attitude
DIAGNOSING DRAGGING BRAKES - BT-101
At 06:03 AM 8/12/03 -0400, McKinnish wrote:
>".... 59 MGA. .... after driving 20-30 miles .... brakes smelled hot on the left side of the car. The knock offs on that side were so hot you could barely touch them while on the right side they were barely warm."
Are you sure it's not dragging on all four wheels? If only the left side wheels drag, then you must have a strange coincidence with at least two separate problems. There are no circumstances I can imagine where one problem can affect two left side brakes.
>"I jacked up the car on that side (after it cooled off) and the rotation of the wheels didn't seem to bind, the brake shoes seemed to be adjusted OK."
Letting it cool off has two implications. Either it's temperature sensitive and relaxes with cooling, possibly due to contraction of fluid as it cools. Or it relaxes with time, possible due to very limited slow fluid flow through one or more of the hydraulic lines (or in the master cylinder).
>"I took it out again and came back to the garage and jacked the car up while it was still hot from the drive and the wheels were almost locked down by the brakes shoes at that time. Any idea what would cause that? It's almost like pressure is building in the brake system just on the 2 left side wheels."
If it's not locking the wheels on the right side, you still have (at least) two different problems, but it could be a combination of a few different things. Some of this will overlap on Hydraulics Tech.
The MGA has a single line braking system with common hydraulics from the master cylinder to the 5-way fitting at the brake light switch on top of the frame just ahead of the right side foot well. From there three lines run to left and right front wheels and to the rear axle. At the end of those three lines there are flex hoses. On the rear axle there is a T fitting to split the flow to the two rear wheels.
The common problems arise with aging hoses that deteriorate and collapse inside to block the flow. These then act like check valves, allowing passage of high pressure fluid from the master cylinder to the wheels, but restricting the low pressure return flow. This commonly causes the lock up of one front wheel, or both rear wheels. To check for this condition, when the brake is dragging, jack it up, confirm the drag by turning the wheel. Then open the bleed nipple to relieve any internal pressure. If the caliper immediately releases and the drag goes away, then there is a restriction in the hydraulic line, usually a bad hose, sometimes a crushed steel line (more common on the rear axle). There might otherwise be a problem with the master cylinder not relieving pressure, but that would affect all wheels at once, not just a single wheel.
If the car was towed by picking up the rear end, or tied down on a flat bet truck or trailer, it is fairly common to have the steel brake lines on top of the rear axle smashed flat and totally cutting off flow to one or both of the rear wheels. In that case the wheel with the smashed line will have no brakes at all. The same condition is extremely rare for the front brakes, as those lines are better protected on top of the frame.
There are a couple of possible problems for the master cylinder that can cause it to build pressure and not release. This would generally cause lock up of all four wheels.
One problem is having the push rod adjusted too long so the piston is not allowed to return all the way to the rest position. Cure for that is fairly obvious, shorten the push rod. Adjustment of the master cylinder pushrods calls for a tiny bit of free play. With the pedals full up, turn the push rod to lengthen it until there is no clearance and a small amount of binding in the thread. Then turn it the other way to shorten it just enough that you can tell that there is a little bit of wiggle or free play, and tighten the jam nut. The pedal should then have about 1/8" of free play where you can press it with one finger until you feel the resistance when the pushrod begins to move the piston in the master cylinder. This is why the pedals themselves need to have small return springs on them, to pull the pedal that last 1/8" to the top stop to prevent drag or rattling.
Another (more rare these days) is for a fresh repacking kit to have rubber cups that are a little to long and do not quite clear the fluid return hole to the reservoir when the piston is at rest. I have heard rumors that this could also happen if there was a change of fluid type that might cause some swelling of older seals. The fix for that is to remove the end cover from the master cylinder, install two paper gaskets, and cut the larger holes in the gaskets large enough to completely clear the bores of the master cylinder. This allows the pistons to retract a tad beyond the end of the body before hitting the end plate, and that is enough to allow the seals on the pistons to clear the fluid return port into the reservoir.
Another problem that will disable brakes on one wheel only is a corroded and jammed slave cylinder at the wheel. This is fairly common for the rear brakes with only one single acting cylinder made of aluminum, incorporating the parking brake lever in the side of the cylinder, and with end and side boots that can fail and allow water or dirt into the bore. These problems are more prevalent for cars with aging rubber parts, or cars that have been sitting idle for a long time.
Another problem that can cause rear brakes to drag or lock is a sticky pull cable for the hand brake. There are two cables there. When the front main cable drags it will affect both rear wheels, and they will either both drag or both have no parking brake. When the rear cable running across the rear axle drags it will affect the right side rear wheel only, and it will either drag or not operate.
Last concern is proper adjustment of the brakes, especially the rear brakes. The brake shoes should be adjusted to minimal clearance first. Then the hand brake cable should be adjusted for minimal travel of the hand lever (maybe 4 or 5 notches of the ratchet pawl) but loose enough to allow full release and a little slack in the cable at rest. When the shoes wear the hand lever travel increases. If you shorten the cable before adjusting the shoes you can cause a problem with the cable being too tight and causing the shoes to drag, also possibly causing a spongy feel for the pedal and reduced braking effect if the hand brake cable is taught when the foot brake is applied. Always adjust the brake shoes first, then the cable, if it still needs adjusting.
Also if the rear brake adjustment is too slack, like it hasn't been adjusted for a long time, or there has been enough wear of the shoes to allow large travel, then the rear brakes may not work at all. With the hand brake lever in the side of the slave cylinder, the slave piston has only a short travel capability before it hits the lever and cannot bring the shoes into full contact with the drums. This then disables the brakes at that wheel, and it might happen to one or both rear wheels.
Back at the front wheels, the 1600 car with front disk brakes is prone to having a caliper dragging when the seals get old and loose their flexibility, or if the outer dust seal fails and allows water or dirt to enter the area between the piston and the cylinder wall. Cure for these problems is to rebuild the caliper with new seals, and maybe replace a piston if it's corroded.
So, to account for more than one wheel locking up, I would think there is a problem with the master cylinder not fully releasing the pressure. If you drive the car and encounter this problem again, then while it is still hot, open a brake bleeder nipple at any wheel (fronts are easier to reach) to relieve the pressure. If that releases the brakes at all wheels, then the problem is in the master cylinder itself. If it releases the brakes at only the one wheel, then the problem is likely to be a collapsed hose, needing to be replaced.
The big question then is how you can have more than one wheel locking up with hydraulic pressure, but not all of them at once. If the rear hose fails the two rear wheels can lock at the same time. A front hose failure affects only one wheel at a time. Master cylinder affects all four wheels. Locking the left side wheels requires a strange combination of failures.
If there is nothing wrong with the master cylinder, then I would suspect failed hoses at left front and at rear axle, with some additional problem that inhibits operation of the right rear brake, such as a smashed steel line or shoes out of adjustment, or corroded and non-functional slave cylinder. Alternately there might be a smashed steel line at the left rear that allows passage of only a small amount of fluid at high pressure, and is extremely slow to bleed down to relieve the pressure.
If there is something wrong with the master cylinder that allows pressure to be retained in the line, that would normally lock up all four wheels. In that case you need to figure out why the right side brakes are not working at all.
First order of business might be to jack up the car and have a helper apply the foot brake while you check to see if the wheels can be turned by hand (all 4 wheels individually). Each wheel should lock up when the pedal is depressed, and should immediately turn free when the pedal is released. Assuming the hydraulic brakes work at all four wheels, then of course they also need to release when the pedal is released. Have the helper pump the pedal a few times in quick succession and then release it, and the brakes still have to release immediately. If one wheel does not release, then open the bleed nipple. If that relieves the pressure to free the wheel, then install a new hose.
Then set the hand brake and see if it locks up both rear wheels. And release the hand brake and see if it releases both rear wheels. If not, then you have to find and fix that problem there before anything else.
When the hydraulic brake works and releases at all four wheels, and the hand brake works and releases both rear wheels, then you can drive it again. If the problem still persists, then it will likely be locking up all four wheels, and you need to fix the master cylinder.