Author Topic: soft brake pedal 1971 Boss 351  (Read 126 times)

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Offline old 429

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soft brake pedal 1971 Boss 351
« on: July 13, 2018, 12:42:18 pm »
Hello,
after I replace my master cylinder , front brake hoses , proportioning valve , I have no pressure on my front brake's .

From start I must push 3-4 times before I get some pressure , when I stop pushing the brake pedal , the brake fluid come back to the master cylinder more than usual.
The new hoses dont  widened when I push the brake pedal .
It looks for me there is still a airbubble air into the front system , I already spend more than 1 gallon brake fluid whit trying bleeding the brakes on several way's  but no good result.

Just order a rebuild kit for the master and proportion valve , and go look if the rebuild it myself original parts are better.
Any other suggestions what to do?

Never have this problem before .
Simon

Offline crossboss

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Re: soft brake pedal 1971 Boss 351
« Reply #1 on: July 13, 2018, 01:57:09 pm »
Sounds like a possible master to booster rod adjustment issue. Loosen the master from the booster, and pull slightly apart, and slowly adjust the the rod out (longer) an 1/8 to a 1/4 turn at a time. Reassemble the master to booster and re-bleed your brakes. PS--you may have to do this a few times in order to get the correct rod distance. DO NOT turn it more than I suggested, or your brakes will lock up and not work.
<My old heap Boss 302…aka S-B Racing!
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1969 Mustang Fastback/FOX chassis, 5 speed, 4 wheel discs, with a modern Can-Am 494 (Boss 429), Kaase headed, intake mounted with a 1425 cfm 'B' Autolite Inline carb, ala Trans-Am style…Whew!

Offline svo2scj

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Re: soft brake pedal 1971 Boss 351
« Reply #2 on: July 13, 2018, 02:36:18 pm »
Or you have your safetyplunger inside the proportioning block PINNED up and only getting pedal to the pin (which will never give pressure to front lines)

Mark
Mark Haas
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Offline old 429

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Re: soft brake pedal 1971 Boss 351
« Reply #3 on: July 13, 2018, 10:00:48 pm »
Crossboss , what must be the clearance from the booster rod to the master ?
now it's arround 1 mm

Mark,  what do you meen whit PINNED up ?
Is there  a proportioning valve problem ? (aftermarket part)

 After pushing the pedal  fast 3-4 times I have pressure to the front lines and my pedal comes higher .

 In the past I replaced  the master and proportioning valve (rearbrake's blocked)  .
 Last week I replace the front hoses , now  I have this soft pedal problem.
 I remount  the distribution blok brake lines as how they are before.
 one brakeline to the rear was replaced by a new one  by a former owner
 No strange bents into the old brake lines .

 BUT maybe the brake lines are wrong mounted  in the past.

And that the brakes are worked fine before was a combine of failures that solved the problem by itselfs ?

Has anybody a picture and discribtion from the routing from the brake lines ?


Crossboss and Mark , both thanks for your reply.
Simon
« Last Edit: July 13, 2018, 11:06:33 pm by old 429 »

Offline svo2scj

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Re: soft brake pedal 1971 Boss 351
« Reply #4 on: July 14, 2018, 04:54:32 am »
Well see if this helps.  My question on the Pinned (above) is if the piston is "centered" in the valve.   IS THE LIGHT ON?
Mark Haas
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Offline old 429

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Re: soft brake pedal 1971 Boss 351
« Reply #5 on: July 14, 2018, 10:12:29 am »
Thanks for the photo's , I go check my lines.
Also found some info on the internet but is different as on your photo's.

The warning lamp is not ,,ON" because the conector inside the switch came out , poor quality aftermarket part.

regards
Simon

Offline old 429

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Re: soft brake pedal 1971 Boss 351
« Reply #6 on: July 14, 2018, 12:00:26 pm »
The brake lines are mounted the same as  from the photo's from Mark .
BUT, I just found out that the warning light switch is leaking brake oil.

I think inside there is something wrong , oil leaking from the front brakes circuit to the rear brake circuit treu the switch rod.

Already ordered a rebuild kit from NPD .

Simon




Online Orangegrower

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Re: soft brake pedal 1971 Boss 351
« Reply #7 on: July 14, 2018, 12:24:24 pm »
The hole for the warning light switch should be a dry hole. Something in that switch is amiss.
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Offline crossboss

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Re: soft brake pedal 1971 Boss 351
« Reply #8 on: July 15, 2018, 09:01:43 am »
Also, forgot to mention check that your master is not leaking brake fluid into the booster. IF you are going to re-set the pushrod length anyways look for fluid in the master since you have it apart. As I mentioned, the push rod clearance will be a set and trial (maybe a few times) for maximum effectiveness. Remember, do not over do it…adjust an 1/8 to 1/4 turns out at a time. Keep us posted.
<My old heap Boss 302…aka S-B Racing!
Current lifelong projects:
1969 Mustang Fastback/FOX chassis, 5 speed, 4 wheel discs, with a modern Can-Am 494 (Boss 429), Kaase headed, intake mounted with a 1425 cfm 'B' Autolite Inline carb, ala Trans-Am style…Whew!