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November 13, 2016


The TR6 cars had a conventional hydraulic brake system, but also a hydraulically actuated clutch.  The brake system has dual circuits, with the front and rear hydraulics separate.  Though there is only a single brake master cylinder, it effectively has two separate piston chambers.  Even a total loss of fluid in either the front or rear brake circuit will not affect the other circuit, though braking effectiveness will be compromised.  Any significant pressure difference in the two circuits during braking will be sensed by a Pressure Differential Warning Actuator (PWDA), which closes an electrical switch in that situation.

Since some of the brake and clutch hydraulic components are mounted to the body, these systems couldn't be finalized until the body was back on the frame.

First thing up was the PDWA.  This is a pretty simple device consisting of a sliding shuttle inside a bore.  The ends of the shuttle are exposed to the pressures in the two braking circuits, and the shuttle carries a seal at each end that keep the circuits separate.  A sufficient imbalance in pressures will move the shuttle toward the low pressure side, which actuates the switch.

My PDWA was simple to renew, just having two O rings and a copper seal for the end screw.

The PWDA was mounted in its rightful place near the master cylinders, and the hydraulic lines form the front and rear brakes were connected. To help ensure that the lines don't get misconnected, the fittings on the PDWA and the master cylinder for the front brake circuit are different from those for the rear. (3/8-24 for ther rear, and 7/16-20 for the front.)  It's still possible to screw this up though if you make your own lines.

Next, I collected the rebuilt pedal box off the shelf, and screwed into place on the top of the driver's foot well with its nine fasteners.

With the pedal box in place, the rebuilt  brake servo could then be mounted along with its thick aluminum spacer.  A little strip-caulk seal didn't seem like a bad idea.

Hmmm, this doesn't seem quite right.

Well, a step backward and a lesson learned.  This looks better.

When I unpacked the new master cylinder I ordered, I realized it wasn't the TRW unit I specified, so I had to send it back.  In the mean time I turned to the clutch master.  It's a simple device and came apart easily.  I powder coated it because...well, because I can.

The original clutch hydraulic connection was a combination of a short 1/4" hard line and a flexible plastic line to accommodate relative movement between the engine/transmission and the body.

The plastic line was still intact, but I decided to replace it anyway.  I must have beel feeling frisky at that instant, since I sprung for the totally unnecessary "uprated" braded stainless flex line.  I do like the looks of the stainless line compared to the plastic.

About that time, the new brake master arrived.  I dutifully read all of the warnings on the packaging and on the reservoir cap.

Now, looking past the mildly humorous DOT3/DOT4 disconnect, I focused on the specific warning against silicone (DOT5) fluids.  I ignored the threat of a voided waranty, since voiding warranties is sort of a hobby for me.  My inquiring side wanted to know why TRW was so afraid of DOT5.  I had done some research on DOT3/4/5 fluids already, and was way past many of the myths propagated on the Internet echo chamber.

Take the claim that DOT5 is incompatible with some brake system rubber components, for example.  Under US Federal regulation, a "brake fluid" is defined thus:

"Brake fluid means a liquid designed for use in a motor vehicle hydraulic brake system in which it will contact elastomeric components made of styrene and butadiene rubber (SBR), ethylene and propylene rubber (EPR), polychloroprene (CR) brake hose inner tube stock or natural rubber (NR)."  [49 CFR 571.116]

So it appears that any maker of brake fluid who wants to claim compliance with US Federal standards (and every bottle I've seen does) will have to ensure compatibility with those rubber compounds.  On the other side of the coin, any maker of brake parts for new or old cars would be crazy to use any rubber component not on that list.

This agrees with the large majority of anecdotal evidence I've read about the use of DOT5 fluids--in new systems or after good flushing, most reports indicate that there is no compatibility problem.  In fact, a liesurely stroll through the rest of the federal brake fluid regulation shows that DOT5 is superior to DOT3 and DOT4 in nearly every measure.

There is one area where DOT5 may have worse performance than other fluids.  Under certain conditions, DOT5 fluids can retain air, which can increase its compressibility.  This is apparently the reason that DOT5 fluids are not recommended for use in ABS systems, which can generate air bubbles in the fluid.  Air retention in DOT5 fluids can increase at higher temperatures, but in non-ABS systems, the effect is reportedly minor, except possibly in racing situations.

There is a wonderful tretise on brake fluids on our own Buckeye Triumph site.  I've seen it referenced all over the Internet.  It dives into the compressibility issue to a good depth.

So this is all good stuff,  but it doesn't definitively answer the question about TRW's bias against silicone brake fluids.  In the end, I decided not to spend any more time on it.  I went to get some brake fluid.

Besides the high I get from trashing a warranty, my main motivation in wanting to use a silicone fluid is more pragmatic:  It doesn't damage paint.  Bleeding an entire brake system from scratch can be messy, with a good chance of a leak or two.  

So, with that bridge crossed, and the warranty card carefully placed in the circular file--onward!

I bolted up the master cylinder and connected it to the pedal.  
Since the master cylinder was empty, system bleeding can go faster if the master is bled first.  Often called "bench bleeding", I actually did it on the car.  I just ran hoses from the outlets back to the reservoirs and pumped the pedal until the fluid ran without bubbles.  Then hooked up the lines.

 Then, to bleed the lines and slaves, I thought it would be easier if all four wheels were off.  This made it simple to make a few orbits around the car, hitting each corner in turn. Since I'm a solo act most of the time, I used the vacuum method of bleeding where a small hand operated vacuum pump pulls fluid from the bleed screws.   It works very well, the only downside being that the open bleeder can suck a little air around its threads.  After a few trials, it was easy to distinguish the foamy, sudsy look of the leaked bubbles from the larger bubbles of air from inside the system.

After fixing a couple of small fitting leaks, the clutch and brake systems seem to be tight and functional.  It's hard to evaluate the brake pedal with no seat to sit in, but it appears to be pretty firm.

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