QuestionHi Howard
I own a Triumph TR5 1968 (RHD) which is driven occasionally, but does spend most of the time in the garage. I have noticed over a period the brake pedal pressure had reduced make the pedal a little spongy. I took the car into be serviced and requested the brakes be bled to address the issue. My mechanic bled the brakes but after picking up the car I found the brake pedal remained soft. I reported the the issue to my mechanic who suggested the master cylinder needed to be replaced. This was done and the brakes were better but still seemed a little soft but ok. The car has spent approx 12 months in the garage with the odd weekend drive, and the brake pedal was again soft. I have bled the brakes manually using the recommended procedure and bleeder sequence (rear first, furthest from MC etc) I got air bubbles from the rear brakes but the pedal didn't fall away when the bleed nipple was opened, consequently little fluid appeared to exit the nipple but some air definitely did on both brakes. On the front, I bleed the brakes numerous times expelling all air and pumping fluid through to fill my bled jar. I made sure the MC was topped up throughout the process. I found the the pedal pressure is still soft, front brakes are engaging but the rear do not appear to engage, so I adjusted the rear shoes, however it was obvious not adjustment was required as they were one notch off as recommended. I jacked the rear and applied the the foot brake but the rear will not engage but the hand brake will hold the rear wheels quite well. I have repeated the above process several times without any improvement. There is not evidence of brake fluid leaks at any wheel, and the brakes were bled and I assume checked relatively recently. The only other consideration which may be relevant, is I used some brake fluid dott3 I had on hand which I assumed would be ok to top up for bleeding, assuming any issue from this decision will only be a problem if the brakes get hot in future. I plan to replace the fluid once the bleeding issue is resolved.
Ps, thanks again..I haven't bothered you since 2010 so I was hoping you could assist.
Thanks Robert
AnswerHi Robert,
At times you can run into a brake problem that nothing seems to correct it. Even in dealerships we have a problem.
For this test you need a set of bleeder valves that fit the line fittings on the master cylinder. Some auto parts stores have a bleeder valve assortment. Also check the front caliper bleeder valves to see if they are the same side as the line fittings in the master cylinder.
When you have two bleeder valves remove the two lines from the master cylinder and put the two bleeder valves in place of the lines. Then pump up the brake pedal and bleed at both valves. You should quickly get a rock hard pedal. When you have that hold a very light pressure on the brake pedal for a minute or two. The pedal should not go down at all. If you don't get a rock hard pedal and / or the pedal goes down with very light pressure, the master cylinder is definitely bad.
If however you get a rock hard pedal and the pedal will not go down with light pressure on the pedal then there is nothing wrong with the master cylinder. It means the problem is further out. A simple test to isolate the problem is to only install one brake line and leave the other bleeder valve in place and then pump up the brakes again and bleed the brakes on which ever you have the line connected.
If it is the front line you have connected have someone pump the brake pedal while you watch very closely at the relationship of the disk to the caliper to see if you see any slight flexing of the disk. There should be NO movement noted on either side. If you see movement it means one of the pistons is stuck and not moving and the opposite piston is pushing the disk over to meet the other piston. Another thing to check is to hold the flex line while someone pumps the brakes to see if you feel the line expanding.
If the brakes feel good with the front connected then switch the bleeder in the master to the other line fitting and thus blocking off the front and now only operating the rear brakes. The rear brakes alone will have more travel then just the front. But the same is true. If you don't get any fluid but have a good pedal then check the rear flex line and try to bleed at the front of the flex line as a swelled shut flex line will stop fluid. If you are able to bleed at both rear wheel cylinders then note the amount of pedal you get and then pull up hard on the hand brake and check the pedal travel again. If you gain a lot of pedal by pulling up on the hand brake then you need to look at the shoes and adjusters.
Air can not enter a brake system except through the master cylinder. If you have a vacuum booster do all the tests without the engine running. Also check the booster for fluid in it.
I don't like to use DOT3 fluid at all even though they claim that DOT3 does not harm hydraulic systems any more. England owned the majority of the natural rubber in the world when your car was made and DOT3 fluid destroyed natural rubber back then. So it must never be used in British cars back then. The trouble is that unless you know for sure none of the seals are from new /old stock, you have no way to know for sure the fluid won't harm the system. So always use DOT4. DOT4 is a better grade of fluid anyway as it can stand higher temperatures.
If you have just rebuilt the front calipers or you have just put in new pads, there is another problem that causes a soft pedal.
Calipers (except some old Jaguars# don't have any method to retract the piston away from the pads when the pedal is released so the manyfactures have designed the seals to flex when the pressure is applied and when the pedal is released the seals themselves pull the pistons away from the pads. The trouble with this system #sometimes) is that the seals over work and pull the piston so far away that when you hit the brakes, it takes two pumps of the pedal to get brake pressure.
If this is the case here, to test it you need to remove the pads and put in place a worn set of pads and then pump up the brakes and then remove the thin worn pads and carefuly pry the piston just enough to have to force the new pads in. If you pry the pistons in far enough that the new pads slid in freely you need to repete the process so that all the pads fit tight. Now if you rotate the disk it will be hard to turn and you should have a good pedal. But if you drive the car like that it will burn the surface of the pads and overheat the disks. So the procedure is to drive the car harshly in a circle in each direction or make harsh "S" turns at slow speed. This action will force the pistons back into their cylinders just a slight amount and not have the seals over flexed. A quick test after the turns or circles is to jack the front up and hand rotate the wheels to see that they did free up.
let me know,
Howard
