Low Brake Pedal

[RDKeysor]

I recently bled the brakes on my '60 BN7 and came up with pretty dark fluid of unknown vintage. Forum members suggested the coloring was likely from rubber deterioration and the master cylinder (MC) likely the culprit. Subsequently, I put a rebuild kit in the MC and pretty thoroughly flushed out the system. I again bled the brakes conventionally, securing a good pedal, but experienced variations in pedal height. Forum members urged me to get a new MC. I installed a new MC, again flushing the system (including removing and cleaning the reservoir), and went around the system three times to insure all air was removed. By the second bleed we obtained a good, firm but low pedal. Lower, I think than I had at the beginning. My rear drums are adjusted to the recommended tension. I am now using NAPA Dot 4, which is the fluid used by our pretty experienced club members.

There are no leaks in the system, as the car has been on jack stands with the wheels off during the repeated bleeds.

I might add that the new MC appears identical to the original unit. Interestingly, or puzzlingly, the interior and fittings in the original MC looked perfectly OK. Subsequent to installing the rebuild kit, I removed and examining the components. Nothing appeared amiss. Nonetheless I then went to a new MC, as described earlier. So why do I have a low, firm pedal? I haven't measured the movement, but it must exceed 2" of travel before going firm. Unlike the situation when there is air in the lines, pumping does not alter the pedal position.

 

[HealeyRick]

Did you check to see that the bore size in the replacement is the same as the original?

 

[rjc157]

Did you replace the hoses

 

[RDKeysor]

Unfortunately, no and no. I assume the bore is the same, making that estimate because the same supplier sold me a rebuild kit that fit the original MC. I do know that there was a difference in MC bore diameter between later servo-equipped and non servo cars. I did the brake hose line change with my Jaguar, so I may have to give that a try. I think that did help stop a condition where the brakes would drag after pulling up to a traffic light. I bought a laser temperature instrument when I was combatting that condition, thinking that just one caliper was dragging. That didn't prove fruitful. Thanks.

 

[Frameman]

It might be as simple as the viscosity of the brake fluid, it might be too light. Until a few years I was under the impression that hydraulic fluids are not compressible but it turns out that they are. This was discussed on an earlier thread on the forum. The problem was resolved by changing the brake fluid to one with a higher viscosity. To find out where you can get this in your area please check with Dow chemical. They also recommended not using silicone brake fluid because silicone attracts air in the system. This is important to remember because the Healey brake system is a vented system not a closed one.

 

[roscoe]

I suppose there are many different experiences out there but when I did my rebuild, including all 4 slave cylinders and master cylinder I chose silicone brake fluid. I had a few minor seeps for a short period of time but it has been absolutely trouble free and not leaked a drop in the last 5 years. I did put in stainless brake flex lines and have since put front disc brakes on. I'm a fan of silicone if put in an all refurbished system.

 

[bdcvg]

Silicone fluid does not absorb water, which is a good thing except water is eventually encountered in brake systems. Conventional fluid turns dark with moisture, time to change. Silicone fluid floats leaving the moisture to collect in low spots in the lines. If Your car sits for long periods the water starts eating the insides of your lines.

 

[why]

Um, I am not sure about adjusting drums according to a spring tension. I have always just turned the adjuster wheel on each side until you just feel the shoe against the drum. for many different cars and years of manufacture. If you do that and find the pedal now higher, then just back off adjuster one "notch", pedal should be about the same and you are ready to go. If not then problem is elsewhere

 

[steveg]

....but it must exceed 2" of travel before going firm. Unlike the situation when there is air in the lines, pumping does not alter the pedal position.

Pedal travel will be greater than MC piston stroke travel. According the Pegasus catalog, the stroke of Girling MCs is 1.375". Have you pulled the boot back and observed how far down the MC piston goes before coming up solid? If the piston goes down 1.375" you are hitting bottom.
Are you using the stock pushrod or do you have an adjustable clevis? Do you have a booster in the system?

 

[RDKeysor]

I am using the stock push rod that came with the new MC. I had it side by side with the unit I removed, and they appeared identical. There is no booster on this car; series one cars didn't come with them as an option. I have a servo kit from VictoriaBritish--I have already installed the mounting plate--but won't install the unit until, and if, I get the brakes working properly. You have previously suggested not installing the servo as it adds additional complexity to the brake system, that thought registers with me. Here's another wrinkle. The May issue of Healey Marque includes an article by Mike McPhail regards brake bleeding. Though I can't find where he mentions rear brake shoe adjustment earlier in the article, in the penultimate paragraph he says this: "Since you have the rear brakes adjusted up tight, the pedal should have very little play. If satisfied, adjust the rear brake drums (shoes) so they just barely drag." My rear brakes are set in the normal position, one click off dragging. Mike McPhail's reference to rear brake adjustment is something I've heard before, and I think I will give that a try when I get my water pump installation completed. I will also check the piston movement on the original MC, since I think it is identical to the new one. I'm not dismissing the idea that ultimate piston travel is the key brake pedal travel.