Thread: Brake Booster failure?

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HOTROD......1974 Triumph TR6 with a 5.0 HO

New front brembo disc, four piston calipers, stock drums in the rear,new lines, new proportionate valve, pressure bleed the system...........no air!

But I have very little brakes.

I think it might be the brake booster ( the only thing I did not change or rebuild) ( vac assisted brakes)

Any Ideas?

how does the brake pedal feel, if it is really firm but lacks power assist you might check the o-ring that goes between the master cylinder and the booster, also another possibility to check before replacing the booster would be the check valve were your vacuum line plugs into the booster, you can pick these up at your local parts store for about $3.00 much cheaper than a booster, hope this helps

Thanks for the reply!

I pulled the motor and trany for the swap, so while I had access, I changed out all the 30 year old lines, fittings, wheel cyl, new pads, shoes. etc.

I did not replace the booster or Master Cyl as I had paper woork from the old owner that It had been replaced in 2000.

I have some pedal, it will slow the car, but it feels like a very bad bleed job, but I have checked and re-checked and re-checked the lines for air........just fluid.

I use a pressure bleeder ( about 15 to 20 lbs ) for one man job.

If the new calipers have a larger volume than the old ones you replaced chances are that a change to a larger master cyl (bigger piston size) will be required. Check with the caliper manufacture to see what they recommend. If this is the case you will also need an adjustable proportioning valve as the bigger MC will overwhelm the rear brakes.

Also another check that would be a good idea is to insure the pushrod from the peddle into the booster is properly adjusted (no slop) and the same with the push rod from the booster to the MC.

Thanks

Summit racing adjustable Prop. valve

It sounds like I need a larger MC.

What would you think of replacing the old calipers and checking sysem?

Expensive. If the calipurs are in good shape and you trust them, a Triumph isnt that much car to stop. Just make sure that you have good lines, a matching master cylinder and all the correct valves, and you should be in good shape. Also, FYI, to check the Vacum boost, pull the feed line right after you shut off the car. If it hisses or pops, your in good shape.

Thanks for the tip.

I'll check the vac line. ( nice trick )

True, there is not much car here, but by swaping in a 5.0 HO, I tripled the horse power, so I want to improve the brakes.

The single piston calipers were problems areas on the TRs, so I installed four piston units and Brembo disc.

I installed a prop. valve so that later in life I can install rear discs.

Sounds like a larger MC is needed.

Someone noted that it might be due to changing over to DOT 5 fluid.

WHY?

Heres a little bout DOT 5

DOT5 does not absorb water and may be useful where water absorption is a problem.

DOT5 does NOT mix with DOT3 or DOT4. They also maintain that all reported problems with DOT5 are probably due to some degree of mixing with other fluid types. They said the proper way to convert to DOT5 is to totally rebuild the hydraulic system.

Reports of DOT5 causing premature failure of rubber brake parts were more common with early DOT5 formulations. This is thought to be due to improper addition of swelling agents and has been fixed in recent formulations.

DOT5 is compatible with all rubber formulations.

DOT5 doesn't eat paint.

Careful bleeding is required to get all of the air out of the system. Small bubbles can form in the fluid that will form large bubbles over time. It may be necessary to do a series of bleeds.

DOT5 is probably not the thing to use in your race car although it is rated to stand up to the heat generated during racing conditions. The reason for this recommendation is the difficult bleeding mentioned above.

DOT5 is a good choice for the weekend driver/show car. It doesn't absorb water and it doesn't eat paint. One caveat is that because it doesn't absorb water, water that gets in the system will tend to collect at low points. In this scenario, it would actually be promoting corrosion! Annual flushing might be a good idea.

In other words, it would be more beneficial to got with DOT 3 and play it safe. Considering the Age of your car, the system was probably not built for use with DOT 5 fluid.

OK, DOT 3 it is. We'll make the switch back and I'l post an update.

Thanks for all the info!

Brakes fixed!

It was not a DOT 5 issue, is was not a leak, it was the prop.valve.

Someone ( family crewmember....kids," what does this knob do?" little fingers ) changed my ratio to 0 to the rear putting all pressure to the front.

Changed prop.valve back, bleed the brakes......WOW! WE HAVE PEDAL

Thanks for all the insight!