RPMs increase when brake pedal pushed, vacumm ?

[n/a]

Hello everyone, my 66" 250 sl (euro) seems to run fine (idles at 600rpm) but the rpms increase about 150-200rpms when the brake pedal is pushed. I assume this is a Vacumm leak from the Brake Booster? Can anyone help me pin point the problem. Thanks :?:

[rwmastel]

Sure.  First make sure it's a vac leak by removing the hose and plugging the connection at the intake manifold.  You will have "manual" brakes now.  While in park (auto) or out of gear (manual), press the brake and see if the RPM changes.  Then do so when driving very slowly.  Remember, "manual" brakes!  Does RPM rise again when braking?  If so, then I don't know what your problem is!  If not, then connect the line again and repeat the tests.  If it rises with the line connected then you have a vac leak because the extra air is making the mixture lean and increasing the RPM.  It's either a hose or a booster, so switch the hose first since it's the cheaper of the two.  My booster I replaced this winter was about $450 so make sure you don't buy one unnecessarily.  You might want to change your brake fluid and check your discs/pads.

The "search" tool on this site works really well.

Rodd
1966 230SL Euro
1994 E420

[n/a]

Thanks Rod i will do those tests ( i hope its the hose!). Sorry i did not see the search option, Thanks again
Mauricio

[ricoled]

I had this problem a month ago. If it isn't the vacuum hose leaking, then check where the master cylinder is mounted to the booster - listen for slight hissing around the flange for possible air leaks - each time pedal is depressed. There is an O-ring here that could leak after some time. However, the reality is that 90% of similar cases  are fixed via replacement of the brake booster.  No repair kits available and yes, they cost around $450 USD.  Part No. 000-430-55-30.  Good luck!

'68 280SL
'82 240D
'90 260E

[Longtooth]

Must be an epidemic... same problem with rev increasing 400 rpm when idling with brake pressed started 2 days ago.  I'd already done the checks Rodd suggested,  so am down to hoping the seal ring between master cyl. and booster is leaking.  I disconnected the vac hose at the booster intake, plugged the hose and pressed on the brake... no rpm increase.  I also thought it might be possible that the venturi was on the hairy edge of opening at idle (800 -900 rpm) so that even a very slight vac leak would kick it open... or that the venturi valve had gotten 'stuck' from full return to closed position, hence even a small vac leak would open it.  That wasn't the case either though.  I'm 99% resigned to ordering the booster anyway... the damned thing's only 36 original years old though, so I thought it should last another few decades being that it's lasted this long already.

BTW, a soft press (not hard on the brake) doesn't induce any rpm increase at all... not even a hic-cup of one.... but beyond a certain point the rpm just cruises on up to 1400 independant of how hard the brake's pressed.  This makes me suspect the leak in the booster is binarily position dependant... some distortion level opens the 'hole' somewhere inside the booster... below some threshold the 'hole' remains closed ... but I don't have any diagrams of the internals so can't really be sure of the exact mechanism.

Longtooth
67 250SL
95 SL500

[hands_aus]

I remember reading some time ago about the adjustment of the ecentric bolt on the brake pedal.

Aparently if the pedal is out of alignment, the seal of the booster can be broken allowing air into the booster.

Could this cause problems like we are seeing here?

Bob(Brisbane,Australia)

[Longtooth]

Brake Booster Repair Co. FOUND!

I asked the restorer friend of mine if he knew of a source for another original T-51 booster that was still operational or whether he could get me a deal on price of new.... $400 or $450 isn't small change unfortunately, though I'd shell it out in a second if that was my last option.

He phone a message with "Call xxx-xxxx... ask for Mike, they repair the original brake boosters --- new internal rubber.  Tell him I gave him your name.... he might give you a price brake on the restoration (repair)."

I called Mike.... asked him if this was for real... did they cut and reweld or how did they repair the sealed unit?  He said they undo the original joints and redo them... can't tell it's been done.  I asked where they got the internal parts & rubber/whatever... he said some are purchased, some they make themselves. "Rubber parts?" I asked... he said yes.  I asked if he knew of others somewhere in the states that do these repairs... he said he didn't know anybody else, but why did I ask?  Apparently he didn't think this was a big deal... so maybe it's easier to get these repaired than some may think.

Anyway, Here's the vitals if you need to get yours repaired.  I'll post another message after I get mine done and tell you how it worked out --- appearance, function, etc.

Power Brake and Exhange
260 Phelan Ave.
San Jose, CA
tele: (408) 292-1305  

Price: $172.50 + tax
Turn-around 2 days.

BTW I only live 1/2 hour from this location.


Longtooth
67 250SL US #113-043-10-002163
95 SL500

[waqas]

Longtooth- did you ever use these guys for rebuilding your booster? What were your impressions?

I'm almost resigned to replacing the booster in my '67 250SE coupe, as I hear the tell-tale hissing sound (with increase rpms) whenever I press the brake pedal. The vacuum hose seems ok. Can anyone think of any other reason? (seal between master and booster perhaps? And would that still create a hissing sound from INSIDE the car?)

Thanks and kind regards,

WAQAS in Austin, Texas

[waqas]

Whoops -- never mind. I just found the post in http://index.php?topic=2067

WAQAS in Austin, Texas

[waqas]

Anyone know how to tell if I've a T51 booster or not? I believe they were installed in all w111 250SE coupes, as well as pagodas... but I'm not sure. What other boosters were in use across the MB line back then? Thanks in advance.

WAQAS in Austin, Texas

[Vince Canepa]

wagas - Power Brake Exchange was recommended to me some years ago by Paul Russell and Company.  I didn't use them since I found another booster, but that sort of recommendation should be enough to trust them.

Vince Canepa
1967 250SL
113.043-10-001543
568H Signal Red
116 Caviar MB-Tex

[German Dude]

Wagas,
I am having trouble with my brake booster as well. I went to a Benz dealership asking which other models used the T 51 brake booster. The guy told me that the T 51 was used in the Pagoda only.

No wonder that darn thing is so expensive.

Good luck with yours.

Juergen
64 230SL #2933, 4sp, signal red, white hard top
2000 320 CDI Station Wagon

[Sven]

I been having the same problem.  I have a 1968 280SL and a 1970 280SEL (parts car only). Anyone know if the booster out of 1970 280SEL is compatible with the SL (i.e., if in fact it is the booster in the SL, with the SEL's work in the SL)?

[waqas]

According to the BBB (68-onwards), there were two boosters in use: the T51/200 and the T51/1801. Anyone know the difference, and if they're inter-changeable without needing to swap other system components?

Also, online parts retailers are listing the ATE 000-430-5530 for around $300 (new ATE OEM part). They refer to the booster as the 9" version.
Any ideas if this will work inter-changeably in cars originally fitted with the 8" version?

WAQAS in Austin, Texas

[Naj ✝︎]

Sven,

 

quote:Anyone know if the booster out of 1970 280SEL is compatible with the SL

ATE part # for the T51/200 booster is 03.6197-0220.4. (Fork type)

Applications include: MB Types: 108,109,111 and 113

naj

68 280SL

[Longtooth]

Waqas,
Yes, I used Power Brake Exchange to rebuild my booster.  I live about 15 miles from them so handed them my booster, but I kept my blue/silver sticker on the original booster though (peeled it off very carefully).  

They took my can apart, replace with new rubber internally, put it back together and charged me $180 incl. tax (walk-in price) at that time (August '03).  I painted it myself again though.... and stuck the my original T-51 sticker back on it.

My costs were:

$16.08 for Bleeder tubes, & DOT 4 Brake Fluid; $183.48 to Rebuild my Booster (incl. Tax)

$190   New ATE Master Cylinder

I cleaned out my brake fluid reservior real well while I was at it.

My symptoms were:
Idle rpm increases 500-600 rpm on Brake Actuation; Caused by Vac. Leak internal to Booster, which was caused by Brake Fluid leak from Master Cylinder to Booster, due to piston vacuum seals on Master Cylinder worn-out + plugged Leak Hole on MC.  

So.... I replaced my Master Cylinder (took mine apart first and found it was shot and couldn't be rebuilt).... with a new one, with same PN as was original for my '67 250SL... that set me back a few though... total cost of the new master cylinder was something a bit under ~$200 ... so overall cost was $396 including T-51 booster rebuild, & new brake fluid.

Not a hint of any problems since.  

fwiw, this place was recommended to me by a restorer of repute.  Their mail-in price is slightly higher.  I'd call first, make contact and explain what you want (and if they still have access to the rubber internals (mine were from the factory mold... don't know where the mold is now though...i.e. who / what firm has it or where the rubber actually comes from.  My thoughts were that it's no big deal to r&r the booster itself... 1/2hrs worth of time +/- (in and out), and at 1/2 the price of new I could do this 2 times if the rubber started leaking... but there was no reason for the booster to fail if my Master Cylinder seal would have remained intact... and that would have remained intact if the Master Cylinder breather hole wouldn't have been clogged for I don't know how long... probably since before I bought the car in '84.  So, I drove the car after brake bleed and brake-line rubber replacements for about 7k? miles and 3 years before it bonked.

fyi... my card for Power Brake Exchange Inc. shows (as of '03):

Micheal Weller

260 Phelan Ave.
San Jose, CA
Ph: (408) 292-1305
pbesanjose@AOL.com

or

6853 Suva St.
Bell Gardens, CA
Ph: (562) 806-6661
Hacket@aol.com  

The shop's in an small industrial area of San Jose... real dump of a place.... brake boosters in volume production rebuild mode for most US built cars, rooms full of rebuilts packaged in retail boxes for shipment to auto parts wholesaler's nation-wide, or at least in Western US. They do walk-in work, but their business in San Jose is rebuilding brake-boosters in volume.  The T-51's not, of course, one of their production line rebuilds, so it's done by one or two guys that know this part and how to do it so you can barely tell the crinks have been redone, & only if you're familiar with the original crinks (whatever they're called that hold the can together).  Anyway, they know what they're doing and do it right.... despite the fact that it's a real industrial dump of a place. Takes a day turn-around unless both of the guys that do this for this part happen to be out (as in my case.... so it took 3 days (drop-off to pick-up).

Longtooth
67 250SL US #113-043-10-002163
'02 SL500 Sport