Power Brake Issues on an AD

[Hampshire]

About a year and a half ago we put this frame mounted booster/master kit in my 1949 AD for my disc/disc setup.

http://www.mpbrakes.com/1949-chevrol...1?filter_name=

MPB was helpful & we ended up we a great setup properly configured with a proportioning valve, etc. Ran it for a season (about 1200 miles) with no problems. The truck went into the shop for an engine swap & frame mods. We had to cut/replace rear brake line and relocate the vacuum to the of the new motor. (6/V8 swap).

Months later we finish up, bleed the system and no brakes when we fire it up. We check the vacuum and it's within spec at 19-20+. We call MPB and per their instructions we first check the integrity of the vacuum line while it's running (no collapses). Still per MPB, we bleed each caliper again and then every joint at the master, etc. Same story - we get some pedal but as soon as we start up the truck the pedal bottoms. Per their instructions, we even try it with two of the outlets at the master plugged - still no dice.

So, we end up paying almost full retail for a new booster (yes, it was over a year old and out of warranty) which I'm now waiting for....

My questions to the forum:

- Do you think it's a faulty booster? I had the feeling the tech at MPB was guessing. He said that seals can dry out with light use.

- I paid (at a discount) nearly double for a replacement - (for what I am seeing a 7" dual diaphragm brake booster go for online at Speedway, etc,). Does anyone know if these are US made? or...

- Is this just a standard GM 7" dual diaphragm booster? Could I have just gone down to NAPA or wherever? I'm not sure what I am paying a premium price for here….plus waiting…

BTW, for now, I have a clamp on the vacuum line and have done some careful country road driving this way. With this Mickey Mouse setup, I have just enough pedal to stop with some planning...kind of like the old Huck brakes!

[dwcsr]

If I understood your issue. No pedal without the booster is a bad master or the prop valve was not centered when you bled it. Blocked off at the master you should have a rock hard pedal without the booster and almost as hard running. The booster is an assist device and you should have usable brakes with or without it that can stop the vehicle safely.

I would center the prop valve with the centering tool a bleed again. check the pedal at the prop vale with the rear and two front lines blocked off, if a good pedal then move on to the rear and bleed the lines make sure the drums are adjusted properly, then move to the front. If no pedal at the prop vale then replace the master.

booster failure is generally a hard pedal when running or not running, not to the floor pedal when running and better when not running. if the pedal goes further to the floor when running then the booster is doing its job. Your issues are elsewhere.

drying out with light use is a junk booster and BS answer, They rot out from not using quality diaphragms and not using a inline filter in the vacuum hose to stop the emission from eating the cheap rubber.

The booster they use is the same as the $45 7" dual on eBay. I purchased both to see the differences and they are the same. Your better off with a early dual 76-77 bronco booster rebuilt by Cardone

[wolffcub]

What they charging you for a replacement booster? I unfortunately had others issues with mine due to a vacuum leak but got a new one for just over $100 cad.

[wolffcub]

if you have brakes without the booster you still should have them with the booster getting vacuum.

I misread your post at first and thought the pedal was being sucked down by its self when the truck was running. Mine kinda did this a bit at first but i noticed my brake return spring was off and the weight of the pedal and arm assembly was helping it pull down a bit when vibrating on the bumpy roads. Hooking up my return spring helped this obviously.

Stupid question but is the arm from the booster properly set to the right length so its pushing in the master cylinder piston enough? Did you have that part of the system back apart for some reason and changed some adjustment?

[Hampshire]

Thank you all for replying -

I'll try to add some clarity:

DWSCR - First, we some pedal with the booster blocked ( just a clamp on the hose) or we have some with the engine off. When we start the engine and the vacuum line is clear we go from having some pedal to absolute zero... I was also wondering about the cost & what I am getting. (BTW, I have 4wheel discs - no drums)

Wolffcub - MPB wanted $195.00 + shipping to replace the booster - instead they have sent it at the "bargain" price of $165.00 shipped.....I'll assume the arm is set properly - this brake system worked flawlessly for over a year and presented no problems during installation for my shop.

[1project2many]

Master ports plugged and no pedal generally means either the master is not bled or the master is faulty. It _is_ possible for air to remain trapped in the master. Bleeding the master on the vehicle can take more than just simple press-release manipulation. I usually judge what's happening by watching fluid / air exiting the ports when a helper is pressing the brakes.

The booster is set up so when there's no vacuum the brake pedal push rod directly actuates the master cylinder. The first piston in the master builds pressure which will then apply the second piston. Air in either chamber makes for a low pedal. It is entirely possible that a change in cab to master alignment is causing the master to be partially applied and preventing the master cylinder pistons from returning fully. Again, checking the volume of fluid exiting the master during brake pedal application can help indicate a problem. When a vendor tells me I have to pay more than reasonable markup for a replacement part because I probably didn't use it enough, I suspect BS and check everything fully. Then I probably don't buy from that vendor again!

[Hampshire]

Master ports plugged and no pedal generally means either the master is not bled or the master is faulty. It _is_ possible for air to remain trapped in the master. Bleeding the master on the vehicle can take more than just simple press-release manipulation. I usually judge what's happening by watching fluid / air exiting the ports when a helper is pressing the brakes.

....I was up above as the helper, so we had a good eye below. We did get pedal bleeding this way, whether by doing the wheels, the master or blocking the ports...though like I said, we went to zero pedal every time we fired it up...

The cab to master alignment suggestion is intriguing, but we didn't move anything even after doing all the work..

Lots of good food for thought from everyone - really appreciate the chiming in.

[thotfulspot]

How much fluid did you bleed? Did it ever run dry. I've had issues like that when it works fine when put on a trailer, but bouncing around during transport moves air bubbles around and end up with no brake when pulling it off the trailer. I usually end up filling the master several times during bleeding to get all the air out. Is the bleeding screw on the top of the caliper?

[Hampshire]

I usually end up filling the master several times during bleeding to get all the air out. Is the bleeding screw on the top of the caliper?

...the bleeders on the rear calipers are, so to make sure all the reservior area is bled we take them off to bleed them

[OrrieG]

Long shot, but are the calipers mounted correctly? Most will fit on either side, but bleeders need to be at the top to get all the air out. If it worked before and you did not change anything then probably ok. I agree with others, pedal going to the floor is faulty MC or air in the system. I went thru two large bottles of fluid to get mine bleed correctly after installing all new lines and MC. Also I am on MC #3 from sitting and got being exercised to keep the seals from shrinking.

[Hampshire]

Couple updates & corrections as we worked on this many hours yesterday...

- rear bleeders are on the sides, so we remove them to tilt the calipers so the bleeder is on the top.

- no proportional valve- I meant residual valves.

- we are going through this methodically and eliminating all possibilities before replacing parts: we bled the system with a power bleeder, and spent a lot of time adjusting the integral e- brakes on the rear calipers. We think this may be part of the problem as it does seem to affect function. Just discovered last night where the brake kit came from (Marty's bow ties in CA) & after looking at the on-line instructions, we have a new process to try (probably what we did a year and a half ago). This morning we are going to try this...

[chevyguyase]

Here are a couple of things to look for. if your calipers are on the wrong side the bleeders will not be on top. You should have good hard brakes with a bad booster, a booster only assists you . Put your foot on the brake pedal and start the truck, the pedal should drop about an inch if the booster is working. Engine off, have some one hold the brake pedal down, remove the master cylinder lid, have the helper release the brake pedal, if fluid squirts up out of the master cylinder you are compressing air because there is air in the system (You can't compress a fluid). A little turbulence in the master cylinder is ok, it's just fluid returning. You can isolate the air problem by carefully pinching off the rubber hoses to the calipers to find the trapped air. I have special pliars I use for pinching hoses. Don't pinch to hard or you will damage your hoses, and don't push on the brakes to hard and don't forget to remove the pliars when your done. (I have found pliars on hoses). Hope this helps.

[Hampshire]

We now have operating power brakes...& the parts MPB sent were returned, unopened/unused.

The brake pedal is even better than we had last year - what was done...

- Started back at the pedal once more & discovered somewhere a bolt in the pedal linkage recently got "displaced/turned around" & that would not allow the pedal to bottom/have full travel...Making it impossible to fully bleed/test. Fixed that and got 2" more pedal travel.

- fixing the above and then re-adjusting the internal caliper emergency brake system as per the originator's (Matt's Bowties) instructions helped improve was already a good pedal (comparing to last year).

- So, with full pedal travel, proper e-brake adjustment and a final power bleed I am now on the road...now I just have to dodge summer storm cells...

Thanks all for respoding - hard to know when you are not there, but you all helped us second think everything and go over every square inch before we replaced anything.