Hello everyone!

I have determined that my brake booster has finally given up; it holds a vacuum but loses it as soon as you depress the pedal even a milometer. (I've already replaced the one-way valve.)

I looked over the forums here, and it sounds like rebuilding one is tricky and can have very mixed results. The one I have is in fine shape (cosmetically), but there's no one around here who can reliably rebuild it (assuming kits are available).

MSA does sell a refurbished brake booster, and I've ordered pretty good parts from them in the past. In addition to a refurbished booster, they offer a 6" and 8" tandem master vac conversion. The links are here:

Refurbished ($189 + core): https://www.thezstore.com/page/TZS/PROD/24-5230

Tandem 6" ($279): https://www.thezstore.com/page/TZS/PROD/24-5761

Tandem 8" ($309): https://www.thezstore.com/page/TZS/PROD/24-5762

I noticed the warning text on the Tandem boosters, and so I called MSA to ask. My car has Wilwood brakes on all four corners (kits 401-8280 and 401-9280 from DP Racing: https://www.dpracing.co/brakes). The system also has a relatively new (but stock) master cylinder and a Wilwood proportioning valve with all the bias turned up (per DP Racing). I've also got 16" rims with 225x45x16 tires (Dunlop Direzza Z).  I've been very happy with the brakes until the master vac failed, but more brake power is better, yes?

I called MSA and asked if I needed to worry about the warning. There were three technicians on hand and two said go with the stock master booster and one said go with the 6" tandem. I'm still thinking more brake is better, and I can just push on the pedal less hard if I need to? The car does see some track time, but 95% of the miles are as a daily driver.  One of my additional concerns about something other than stock is adjusting it all correctly, but again I'm thinking more braking power is better.

So, if anyone has any advice or has tried these tandem boosters or has another solution, I'd greatly appreciate the help!

 

 

 

Edited August 19, 20222 yr by z boy mn

Unless I am missing something, I would expect the vacuum to go down when touching the brakes. That's where the boost effect comes from...

What am I missing?

I'm no brake expert, but I would want a "stock-like" booster, not the tandem.

Not a brake expert by any means, but Power Brake Exchange  out of San Jose CA does a really nice rebuild on the stock booster.

What's the advantage of a Tandem booster, more pressure through the line on a stock master cylinder?

You say that your proportioning valve is running at its limit, so you're not really getting any adjustability out of it... I've read that when upgrading to a toyota caliper you should also change from the 7/8 to a larger master cylinder to make up for the volume difference, do the willwoods also need more pressure/volume?

https://www.classicperform.com/PDFs/BoosterBrakePressureChart.pdf

How did the brakes feel? Where was the engagement point?

Thanks for all your comments and questions...I'm not a brake expert either, not by a long stretch...here's what I'm thinking so far:

@Patcon: while the booster holds pressure until you touch the pedal, as I understand it the vacuum on one side of the diaphragm is what makes them "power" brakes. The atmospheric pressure on the "push" side makes pushing the brakes easier against the vacuum side. But, my booster looses all vacuum the second you even wiggle the brake pedal, so you don't get any assist at all. There's another test, where you pump up the brakes and then turn on the car, adding vacuum (in theory) when you should notice the pedal dips some; mine doesn't. It fails that test, too.

@Captain Obvious: It's pretty tight in there, so I don't know how a tandem booster would fit. I'm assuming the folks at MSA would have mentioned that, though.

@heyitsrama: Thanks for the reference. I'll have time on my side as I'll just keep up the leg workout this fall and take it off when I park it for the long MN winter. So, mailing, etc., is fine. I called a few folks in my area, and they all ship it out, too. The FSM has instructions on how to rebuild it, but I'm not sure the rebuild kits are available to individuals and it doesn't look like an easy job. As for the proportioning valve, I've turned it all the way "up" so that the rear brakes get as much pressure as they can. According to Don, who put the kit together at DP Racing, this is fine as the calipers in the kit for the rear are much smaller, so you don't have to worry about locking up the rear tires. I've never had an issue and the car has some track time. With other kits, or kits where the rear brakes are larger, you have to dial back the brake pressure to the rear to keep things balanced.  I put in a call to Don with this question, too, and I'll ask about the master cylinder, too. Even though mine was replaced by the PO, I've had the car 10 years and it might be worth doing even if it doesn't have any leaks. Finally, the "tandem" has two diaphragms inside, so you get more assistance to the power brakes.

As for how the brakes engage, they feel fine and the pedal is rock hard (another sign). It takes a lot of leg pressure to slow the car down, but it does slow down just fine. The pads are relatively new and the discs have very little wear. The car has about 16,000 miles since the restoration, so the hardware should be in good shape otherwise.

I'll keep folks posted when I hear more...ideas are welcome!

Edited August 20, 20222 yr by z boy mn