TR4/4A Need help with TR4A brakes

[moe_morin]

My 67 TR4A brakes requires pumping 2-3 times to get pressure. After 20 minutes or so, the pedal goes to the floor again requiring pumping. The car has been restored with everything new, except for the brake lines which were cleaned and flushed. New speed bleeders, too. I bled all around twice with DOT 5 fluid until no air bubbles and clear fluid. After bleeding, the pedal has solid pressure for an hour or so. After that, the problem starts all over again. Any thoughts/suggestions will be appreciated.

Question: should the line from the master be bled as well... could it be air bound? Also, there is no backflow preventer installed... would this help with the problem?

Thanks,
Moe Morin

 

[TomMull]

Moe, I assume you have ruled out a leak somewhere in the system? And no, there is no need to bleed the master separately as long as it is pumping airless fluid to the wheels.

 

[trrdster2000]

Moe, check the seals in the master, there were a couple of different ones and the wrong kit will be close but not work. The rear seal can be a problem, but most likely your spring has become disconnected from little tab not being bent into position.

Wayne

 

[TR3driver]

Wasn't there a rash of bad master cylinders a year or two ago? This discussion seems familiar, but I've forgotten the details and didn't bother to save the thread.

Obviously you also need to be certain the MC piston is returning all the way. I've also heard stories of new MCs that would bind and not return completely when used with DOT 5.

 

[TRMark]

Wasn't there a rash of bad master cylinders a year or two ago? This discussion seems familiar, but I've forgotten the details and didn't bother to save the thread.

Obviously you also need to be certain the MC piston is returning all the way. I've also heard stories of new MCs that would bind and not return completely when used with DOT 5.

Exactly what happened when I installed a new MC and converted to DOT5, it bound and would not return when I was pumping it to prime before installation. Although I still do not like the pedal feel of the silicon fluid, I did lock up a set of 205 tires at about 65mph when a deer came out of the borrow pit, using green stuff pads and stock rotors.

 

[moe_morin]

Thanks to all for the input. Sounds like I need to check the MC, but I'm not sure what to do first.

Re-Wayne's comment, how do you check the MC seals and what am I looking for? Same for the spring and "tab".

To Randall's point, how do you check and correct the MC piston, if not returning all the way?

Thanks again.

Moe Morin

 

[trrdster2000]

Moe, I hope you have a manual of some kind. If not, a little research on line will do it. The kits come with new rubber parts and some will fit but will not be large enough to seal under pressure. The rear seal is just a little cup and the spring is held in place by a metal retainer with a small clip that must be pressed down to a ledge for keeping it in place. Very simple and cheep to replace the seals, just having a smooth surface on a old cylinder is a problem, they tend to be ruff on the bottom of the cylinder and require a large amount of honing to get smooth, then you are getting a bit large for the seal to hold up under pressure.

Hope that helped, and check with a Triumph or British car club in your area, they will always be glad to help.

Wayne

 

[TR3driver]

See how far you can move the MC pushrod in and out. .010" to .020" is probably normal; if there is a big gap or the pedal is keeping the rod from coming out against the washer in the end of the MC you've got a problem. If the piston is not coming out, then you'll probably have to open up the MC and polish the bore and/or piston.

The left red arrow in this diagram points to the tab that Wayne mentioned.


The upper illustration shows what should happen when the pedal is released, the piston moves all the way to the right and the clip 6 pulls on the rod (unnumbered) to lift the seal 1 away from the end of the bore. That allows the pressure in the lines to equalize with the reservoir (which is open to the air). If the valve doesn't open, then the system could (at least theoretically) pull a vacuum and suck air past the seals.

 

[moe_morin]

The diagram is very useful. I note its for a Clutch MC, but I assume its identical for the Brake MC.

I'll check the rod travel first, then look at the internals. The body seems to be tapered to the Valve Seal bore. If the bore needs polishing, do I need a special tool?

Thanks a lot Randall and Wayne

 

[trrdster2000]

Moe, I would not recommend you try honing the cylinder, you don't seem to have any expertise in this area and your life my be dependent on how well you do the repair. Take it off and let someone look at it that knows what they are looking at, or send it off, or get a new one. Don't guess at brakes!!!

Wayne

 

[moe_morin]

Wayne... I appreciate and agree with you're concern. FYI... the pushrod travel seems OK to me. I bled all around again and everything was clear with no air bubbles at all. Took a test drive and the brakes are good, just that the pedal seems lower than what I'd like. The pedal feel is solid. I'll keep an eye on it and if something changes, I'll have someone look at it.

Thanks again,

Moe Morin