Brake Line Pressure

[JeffYoung]

I have 500 lbs on the left front, 1100 on the right. Has to be a line issue, right? The entire front circuit runs off of one port on the m/c, so it must be somewhere after the "t" in the line to the left front?

I am assuming the correct thing to do is simply start replacing lines on the left side?

[924Guy]

Is there a proportioning block on the TR8? I seem to recall my TR7 maybe having one... that could be a problem if it's plumbed incorrectly. Otherwise, yes, you've got to review the plumbing... and it may not be a bad time to consider running new fresh clean lines to both fronts!

[JohnRW]

Are you measuring with a single gauge (foot-pressure issues), or two gauges (mechanical gauges lie). ?

Before ripping everything apart, try swapping the gauges side-to-side.

Never trust mechanical gauges. Evil devices.

[JeffYoung]

Vaughan -- yes, the 7s and 8s had the same stock prop valve to decrease rear line pressure. I yanked mine, and installed an in cockpit adjustable.

I also redid all the lines...the whole car...in stainless.....so guess who is at fault for this issue? lol....

John -- I have a single mechanical gauge. I put it in the caliper, and press the pedal down with a board and wedge the board to the cage without releaseing the pedal at all. 500 on left side, 1100 on right was repeatable about 4-5 times, plus I had the car pulling to the right, hard as hell, as the pads wore, plus nearly worn pads on the right and barely worn ones on the left.

Given that do you still think the gaugae is an issue? What's the dual gauge you mention, haven't seen/heard of that?

THanks guys.

[Gary L]

I would respectfully submit that what you're measuring may or may not have anything to do with line pressure or lack thereof. You're measuring caliper output, and my money says the caliper is the problem, not the brake line.

[JeffYoung]

Possible Gary, possible. I thought about that, but discounted it for this reason: I'm measuring pressure out the bleed screw. The gauge is screwed in there. I'm not really measuring clamping for at all.

You still think it could be the caliper? I thought that at first too, but I think I have had this issue for a bit with more than one caliper on that corner.

[JohnRW]

Possible collapsed line...although in a static state (w/o flow) it should equalize...but I've seen them "not equalize", which always confuses me. Maybe a bad flex line...maybe a hard line. Since you're measuring at the bleeder, that pretty much rules out a spanky caliper & clamping force issues.

I just don't trust mechanical pressure gauges for 'repeatability' (am I being repetitive ?). I have two when I'm checking bias/pressures...I check them against each other a lot...because I don't trust them.

[JeffYoung]

I hear ya, thanks John.

Everything is new, so I may have wonkered up a flare or a fitting. I'll start replacing one at a time and see if one of them fixes it (only three lines and one fitting on that side).

[ddewhurst]

Jeff, another second/backup check would be to have a second person set in the car with a bathroom scale between their feet & the brake pedal holding the pounds force on the scale equal wile you check with a torque wrench each front bearing hub/rotor. Make a bracket to get on center or just double nut one stud. As long as you do the same to both sides the test is equal. The results may not be equal as you have stated.

[JeffYoung]

That makes sense...I am certainly "roughing out" the actual pedal pressure applied by wedging the pedal down to teh same spot. should do it as you describe to confirm 100% that there is an issue. Thanks David.

[JeffYoung]

Thanks for the help guys.

Here's what happened. I used stainless lines and fittings EXCEPT for the "t" that branched the two lines off to each front caliper. There, I used a brass t. The stainless lines and fittings collapsed the cone inside the t, blocking off the line to the left front.

Since I couldn't find a stainless t and was trying to make a test day tomorrow, I just replaced everything up front with regular steel lines and fittings and voila, 1100 lbs at both calipers.

Then I dropped the car off the jacks and bent the radiator......didn't leak, whew....

Then I overtightened and screwed up the threads on the oil pressure fitting into the oil pump housing.....doh......no test day tomorrow as I try to drill and tap that sucker.

[JohnRW]

Stainless brake lines = trouble. Similar bad experience a long time ago. Sounded like a good idea at the time, and had a local and economical source of stainless brake tubing and fittings (aka "a friend in the business"). What he didn't explain was that they do all their flaring and fitting on mega$$$ hydraulic forming/flaring machines.

Dopey John tries his normal double-flare tools, and quickly notes he can't get a decent flare since the stainless tubing is TOO FREAKING HARD. The stuff is a bitch to work with.

Still have a big pile of stainless hard line hanging from the rafters here - "free to good home", if anyone is interested.

Another 'sparkly-shiny-hitech doodads aren't necessarily better' lesson learned. Not the final lesson, by any stretch...that's still going on here.

[JeffYoung]

John, amen bro! Biggest mistake I made was going to the stainless lines. Ran about a 50% success rate on getting a good flare. On regular steel, no problem. I've now got the fronts plumbed back with regular steel. Will do the rears shortly. An expensive mistake, sucked in by the gee whiz bangedness of it like you were.